Varnishes and Surface Coatings: Equipment

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The information presented on the Paintings Conservation Wiki is the opinion of the contributors and does not imply endorsement or approval, or recommendation of any treatments, methods, or techniques described.

Authors: Monica Jaworski, Noelle Ocon, David Miller, Frank Zuccari
Date: Submitted October, 1996
Compiler: Wendy Samet

Varnishing Brushes

“With good tools a job is half completed at the start”

(Valentin Boltz 1566 Harley 1972).

Purpose

The purpose of a varnishing brush is to deliver a varnish in suspension to the surface of the painting. The brush facilitates control of the application.

a) Method

Varnish is wicked up through bristles or hairs by capillary action and deposited by pressure and by gravity on the painting.

b) Technique

The general descriptions indicated here are for overall brush application and local application (see fan blender brush, Section g)(4) below, p. 234). Brushes can also be used in a “picking-up” technique (with a badger hair brush or other soft hair brush) or to adjust spray application techniques (See Section 3.b)(5) below).

Types of Fibers

a) Bristle

The term “bristle” traditionally refers to hair from the pig with its conic shape and unique split, flagged ends. Three types exist: hog; boar; and nylon.

b) Hair

“Hair” refers to the fur of animals, usually the tail section. Hair for brushes is characterized by two distinct shapes.

(1) CONIC-SHAPED (POINTED) HAIRS INCLUDE:

(a) Kolinsky

(b) red sable (or weasel tail)

(c) squirrel

(d) mongoose

(e) badger

(2) CYLINDRICAL HAIRS INCLUDE:

(a) ox ear hair

(b) horse-body (pony) hair

(c) goat hair

c) Synthetics

(1) NYLON

(2) POLYESTER

Characteristics of Individual Types of Fibers

a) Bristle

Bristle has a unique structure, with flags or splits at the end of the hair. The flags maximize contact with a surface, providing an excellence in spread and flow. Hog bristle is most commonly used, boar bristle being generally too coarse. Hog bristle is boiled to give it springiness. Sometimes it is bleached, which makes the bristle softer. Some varnishing brushes or artists' brushes are made with bleached bristle (sometimes called White Lily); brushes for the food industry are usually bleached bristle. (Pastry brushes can be excellent for varnishing and are sometimes easier to find than good varnishing brushes.) Hog bristle is durable and resilient. It is used for a wide variety of brushes ranging from artists' brushes to those for household and industrial use.

b) Hair - Conic-Shaped

(1) KOLINSKY (TAIL)

This fine quality hair is extremely strong, springy, and resilient, with excellent pointing and flow properties, and a characteristic snap to it. Kolinsky is characterized by the fine point of the hair with a thickening of the shaft above the base of the hair. This thick area is called a “belly.” Its positioning in the ferrule is critical for a brush to have superb working qualities.

The finest quality Kolinsky comes from male winter tails. The males forage for food while the females nest so they develop extremely luxuriant coats that shield them from the elements. The tails have traditionally been a by-product of the fur industry. With antifur campaigns, the demand for pelts has decreased and the price for tails increased with a smaller supply of high quality fur or hair available.

Kolinsky is generally reserved for the finest quality watercolor brushes. While it would make an excellent varnish brush, the cost for the quantity and length of hair necessary would make it prohibitively expensive. Kolinsky watercolor rounds are excellent for retouching. Flat wash brushes, artists' oil flats, and fan blenders from Kolinsky can be very useful for local varnish application.

(2) RED SABLE (WEASEL TAIL)

Although it has less spring than Kolinsky, red sable is still quite a snappy hair. It has good pointing and flow properties and a structure with a belly like Kolinsky. It is not found as a varnish brush for reasons listed under Kolinsky. Sable is used for making good quality watercolor rounds, artists' oil brushes, and fan blenders. Sable brushes can be very good for retouching, glazing, and local varnishing, and are a less expensive alternative to Kolinsky brushes.

(3) SQUIRREL (TAIL)

Squirrel tail is excellent as a watercolor brush because of the large amount of water it can hold. When properly shaped, it can have beautiful pointing qualities. It is a very soft hair with no spring or body. It is generally considered to be too soft to be used by itself for varnishing and in a flat shape tends to split when wetted. It is sometimes added to certain soft hair brushes made for the application of varnishes and finishes primarily on wood and furniture.

(4) MONGOOSE (TAIL)

Mongoose tail has spring and resiliency similar to red sable and Kolinsky. It is slightly thicker and somewhat wiry with a belly that is higher up than that of Kolinsky or sable. It has excellent flow and handling properties as a varnishing brush and is very good for varnishing fragile or textured surfaces. On small paintings, a “brushing out” technique is possible. It is an excellent hair for a “laying out” varnish technique (no working or brushing out of the varnish). Unfortunately it is rarely found as a varnish brush. Mongoose is most commonly used for artists' flat oil brushes and fan blenders.

(5) BADGER (BODY HAIR)

Very resilient and nicely pointed but has insufficient body for overall varnish application on paintings. Like Kolinsky, sable, and mongoose, it has a “belly” but this is high up toward the tip of the hair. Badger brushes are most commonly used to soften or blend lines or areas in faux finish techniques and to apply glazes because the hair does not leave brush strokes. The word badger is almost synonymous with the word blender. Conservators use badger hair blenders to pick up excess varnish and to matte down newly varnished surfaces.

(6) NYLON

Nylon is used to make varnish or wash brushes. It is a durable fiber and generally found in two grades: smooth (soft) and strong (stiffer). These brushes tend to hold less varnish than natural hairs or bristle and do not have very good flow characteristics. For some applications, these may be useful features.

Fine quality nylon brushes are relatively expensive and may cost nearly as much as a fine quality ox-hair brush.

Coarse nylon house painting brushes with flagged tips in imitation of bristle can be found in paint/hardware stores. They are economical and recommended for use with latex paints and water-based varnishes because the fibers will not be damaged by soaking in water.

c) Hair - Cylindrical Shape

(1) OX EAR HAIR

Ox ear hair has excellent spring and flow. Light colored ox hair is the finest quality. This light colored ox hair is called sabeline. People often confuse this term with sable, assuming it refers to a type of sable, sable-mix, or imitation sable.

Ox hair varnish brushes are commercially available in a variety of styles and are used extensively to make artists' oil and watercolor brushes and fan blenders.

Ox hair varnish brushes are an excellent brush for those who have mastered the craft of laying out a varnish.

(2) HORSE-BODY (PONY)

Horse-body (pony) hair used in Western brushes is a poor quality hair and is generally used for inexpensive, student grade brushes. Pony hair is soft with no body and poor working properties. It is also found as an admixture in some squirrel and “camel hair” brushes. Tails and manes are used to make fairly coarse dusting brushes.

Horse hair is used to make some Oriental brushes with exceptional quality and working properties. This caliber or quality of hair has generally not been made available to Western brush makers.

(3) GOAT HAIR

Goat hair is very soft with no body and is slightly wiry. It is not suitable for varnishing purposes. Goat hair is an excellent hair for dusting brushes as its soft, wiry structure is very effective at picking up dust. Its softness also makes it an excellent hair for buffing surfaces. Some tamping brushes used for gilding are made from goat hair because it is soft and will not scratch the leaf.

Goat hair is also used alone or as an admixture for inexpensive student-quality brushes. It is usually a constituent of “camel hair.” (Actual camel hair is too wiry and unsuitable to be used to make brushes. The terms “camel hair” and “bear hair” refer to a mix of soft hairs.)

(4) NYLON

(See Section b)(6) above.)

Structure

a) Brushes are composed of three main parts

(1) HAIR OR BRISTLE

(for descriptions refer to section above.)

(2) FERRULE OR MOUNT (AND MOUNTING ADHESIVE)

(3) HANDLE

b) Hairs or Bristles

These are cupped into a shape, an adhesive (generally epoxy) is used to secure them and set them in the ferrule. This is then mounted onto a handle and secured with pins or staples.

c) Ferrule or Mount

(1) THE FERRULE NORMALLY SERVES THREE PURPOSES:

(a) to bind the bristles or hairs

(b) to hold the adhesive

(c) to serve as the means of attaching a handle

(2) THERE ARE TWO BASIC TYPES OF FERRULES USED FOR VARNISH BRUSHES

(a) a “cuffed” ferrule, in which the top edge is folded or turned back flat against the ferrule with a flat soldered overlap join on the back

(b) a horizontally ribbed ferrule that has a side-folded crimped join

Of these two types of ferrules, the horizontally ribbed ferrule is more commonly found. The cuffed ferrule, though, is better and safer because it has no sharp edges while the side-crimped ferrule has a sharp edge with a protrusion on the side where the metal is crimped. There have been instances in which the surface of a painting has been scratched by the side crimped ferrule in brushing out the varnish.

(3) OTHER TYPES OF BRUSH FERRULES

(a) seamless molded

(b) hollowed out material (e.g., wood or bone)

(4) FERRULES ARE MADE FROM A VARIETY OF MATERIALS

(a) tin-plated steel

Tin-plated steel is probably the most common material used today for ferrules. It is rust-resistant, durable, economical, and is used for most varnishing and paint brushes and many artists' oil and acrylic brushes.

(b) brass

Brass is generally used only for fine quality water-color brushes. These ferrules are conical and seamless.

(c) copper

Copper is used for traditional-style round sash brushes today. It was used for fine quality flat wide squirrel brushes in the past and may still be used in France. The flat ferrules have a soldered overlapped join on the back and are generally not “cuffed.” The sash brush ferrules taper and are seamless.

(d) aluminum

This is occasionally used for economical good quality brushes though it is mostly used in the manufacture of inexpensive, poor quality, sometimes “disposable” brushes.

(e) leather

Many house painting brushes were once bound with leather ferrules. Sources indicate that these were considered ideal because the leather made for such a perfectly balanced brush. It was also made for stain applications and other purposes for which a metal-free brush was needed. Hair or bristle is generally formed into bundles, either tied or set with adhesive, and secured into the ferrule by stitching with heavy thread. The ferrule is then pinned or stapled over the handle. The ferrule has a stitched seam on the back. (I have seen one round leather-ferruled brush that was seamless–Author.) Leather-ferruled brushes are still made by a small handful of brushmakers.

(f) bone

Tufts of badger hair are pulled through rows of holes in bone which is then mounted with screws into a wood base into which the handle is screwed. Plastic is being used by some manufacturers to replace bone.

(g) wood

Badger blenders mounted into wood are mentioned on page 232. This same type of mount is used for flat bristle, ox, and squirrel for brushes used primarily for faux finishing techniques.

The round wood mount brush is one of the oldest forms and has been produced for many centuries. A length of dowel is hollowed out at one end and the hair or bristle, set with adhesive or tied, is inserted, and then bound with string and/or wire.

(h) synthetic

Nylon and plastics are increasingly used in the manufacture of brushes where a metal-free structure is needed. These ferrules are of a molded, seamless form. These brushes may be desired for book and paper conservation, for stain applications, for use with strong reagents such as acids, and for the food industry because they are easily cleaned and sterilized.

d) Mounting or Adhesive

In the past, pitch, glue, adhesive, and rubber were used to set or secure the hairs or bristles in the ferrule. On old varnish brushes, one may see “vulcanized” stamped into the ferrule. Varnish brushes were generally set with this sulfurized-rubber heat-treated technique. Today, brushes are usually set with an epoxy compound which is more resistant to the wide range of solvents and materials currently used.

e) Handles

(1) MATERIALS

Handles are made from a hardwood, usually beech, and sometimes plastic. Wood handles are found in a variety of finishes: unfinished, oiled, varnished, painted, or lacquered.

(2) HANDLES COME IN A VARIETY OF SHAPES, INCLUDING:

(a) flat

(b) beavertail

(c) round bright flowing type with wood extension

(d) round or rat tail

(e) block

Most varnish brushes used by conservators have flat handles.

(3) DESCRIPTIONS AND CHARACTERISTICS OF VARIOUS HANDLE TYPES:

(a) flat

These taper below the ferrule becoming slightly wider for the handle and usually taper to a point at the end.

(b) beavertail

The shape follows that of the flat handle but is much thicker and slightly rounded. This shape is found on thicker brushes.

(c) round bright flowing type with wood extension

This type of handle has a round metal base that is attached to the ferrule. A round wood extension provides the needed length for the handle. Fine quality chiseled brushes sometimes have this form of handle.

(d) round or rat tail

Oval and round brushes will generally have this type of handle because it allows the brush to be spun during use.

(e) block

This is a flat, squared handle or grip. It is used with brushes which have the hair or bristles mounted into the wood and also some brushes with metal ferrules.

f) The Basic Shapes of Varnishing Brushes

(1) FLAT

(2) CHISELED

(3) OVAL (OR ROUND)

(4) FAN-SHAPED

(5) TUFTED ROWS

g) Descriptions and Characteristics of Various Brush Shapes

(1) FLAT

This is the most common shape for varnish brushes. They come in single, double, or triple thickness (thin, medium, and thick, sometimes referred to as 1, 2, 3 by the manufacturer). Since there is not an industry standard for this, thicknesses may vary widely from one manufacturer to another. A thin, or single thickness, brush could be 3–6 mm in narrow widths and might increase slightly with the width of the brush; a double might be 7–10 mm thick, etc. Cutter, sash brush, mottler, and spatter are terms one may see in catalogs to describe brushes that can make excellent varnishing brushes. Cutter and sash brushes are generally thick; mottler brushes are narrower.

Lengths of bristles or hairs can vary according to manufacturer. In hog bristle varnish brushes, three basic types are found: 1) standard untrimmed; 2) spatter, which is a blunt-end brush composed of flagged and trimmed bristles and is shorter than standard varnishing brushes; and 3) mottler which has trimmed, blunt-end bristles, shorter than a spatter brush. Lengths of mottler and spatter brushes can vary from less than an inch to nearly 2 inches. These are used for manipulating glazes, overgraining, and mottling in faux grain/stone techniques. Though the end is blunt and trimmed, it retains a considerable number of flagged ends. In longer lengths, these types of brushes can be excellent varnishing brushes. Some conservators have described them as very good for brushing out damar.

[Plugs:A Note]

Some thick brushes have plugs inserted between the bristle to open a space in the brush. This will allow the brush to hold more paint or varnish and give it better tapering and flow properties. Plugs also hold the bristle tightly in the ferrule.

For varnishing paintings, plugs are not generally a desirable feature as the brush may hold more varnish than wanted. Plugs are NOT, however, cost-cutting devices to save on bristles and make a brush seem larger than it is. When painting or varnishing large areas, they can be a very useful feature.

(2) CHISELED

Chiseled brushes are generally fairly thick and the hairs are cupped to taper and form a fine thin edge along the bottom. This shape allows for a very uniform flow of varnish without excess flow or ridges at the sides. Chiseled brushes are available in a variety of hairs and hair mixtures: ox hair, bristle, bristle with ox hair, and soft hair mixtures. Bear hair once was used to make a very fine quality chiseled brush. The term “bear” brush is still used though bear hair has been replaced by soft hair mixtures.

These brushes are more often used to apply varnish and lacquer to furniture and boats. The ox hair brush is traditionally used for the application of layers of gesso in preparation for gilded surfaces. For those conservators who have mastered the technique of “laying out” a varnish (no working of or brushing out), the “bear” and ox hair brushes can be superb tools.

(3) OVAL (OR ROUND) SOMETIMES CALLED A POUNDER

These are probably more commonly used for house painting and not frequently found now (Omega and da Vinci brushmakers still make them). The large, long bristled versions of this type of brush generally have an open, plugged center. Louis Pomerantz used this type of brush in a varnishing technique in which he would pour a puddle of varnish in the center of a painting and vigorously brush the varnish out to cover the entire area.

(4) FAN BLENDERS

These are very thin brushes in which the hairs or bristles are splayed like a fan. As the name indicates, they are used to blend paint and glazes, eliminating brush marks. The shape allows for light manipulation of the surface without the edges that one would get with a flat-edged brush. Fine hairs such as sable, ox, and badger are most useful with this type of brush, though it can also be made with bristle or nylon. This type of brush can be extremely useful for localized varnishing to saturate areas of inpainting and adjust areas of varnish that may have sunk in irregularly.

(5) TUFTED BLENDERS

Badger hair blenders generally have the hair bound into bundles which are then pulled through a bone, plastic, or wood plate in which there are rows of holes. Badger has a thick belly close to the tip of the hair. This type of mounting compensates for the extra thickness near the tip of the brush.

Badger blenders are also found mounted with flat metal ferrules like most typical varnishing brushes. Badger mounted with a metal ferrule will generally have a plug. Another type has the badger hairs recessed into wood which also serves as the grip or handle. This is called a “block-style” brush.

These badger blenders are for application of glazes on surfaces and for softening and blending of lines in faux finishing techniques.

Conservators can find these brushes to be extremely useful for picking up excess varnish and matting-down newly varnished surfaces. Since these brushes can be quite expensive, it is a good idea to investigate which type of badger blender will best suit the intended use. Density and hair length can be important factors. Ask those who use “picking up” techniques what brush they use and try out the brush(es), if possible. The block-shaped badger brushes can also be quite useful on smaller paintings and for localized use (and are a fraction of the price of a quality tufted badger blender.)

The Care and Cleaning of Brushes

Brushes should be tapped before using to dislodge any loose or broken bristles or hairs. Dipping the brush in solvent may help to remove dust and other unwanted material.

After use, brushes should be cleaned in the solvent in which the varnish was dissolved. Occasionally one may want to clean them more thoroughly with a commercial brush cleaner that is olein based and then rinse them thoroughly in warm water. This procedure helps to rejuvenate and condition the brush. (This is especially recommended for fine-hair inpainting brushes and will prolong their abused lives.) Care should be taken to “slap out” as much water as possible to prevent rust from forming. Hang the varnish brushes bristle or hair down to dry. Hot water should NOT be used because it can cause shrinkage in the elements inside the ferrule which can result in the brush losing hair or bristle and the handle becoming loose.

After the brush is dry, wrap it in Mylar®. This will keep the brush clean and dust free and help it to retain its shape.

Some conservators prefer to keep each brush restricted to the use of one specific varnish because of residual accumulation in the heel of the brush.

(Editor's note: Some conservators prefer not to clean varnish brushes following each use but wrap them uncleaned in Mylar® for subsequent use with the same varnish. Their theory holds that constant cleaning is harder on the brush than the long exposure to varnish solvents.)

References

[Anon.] 1956. Brushes. In Encyclopedia Britannica. 4:293–5.

Bock, E. 1969. Von Werden der Pinselmacherortes Bechhofen und des dort und in der Umgebung heimischen Handwerkszweiges. Bechhofen: Selbstverlag der Innung handwerklicher Pinsel-, Bürsten- und Zurichter-Betriebe.

Bock, E. 1983. Bürsten und Pinsel. Die vielfältigen Erzeugnisse des Bürsten- und Pinselmacher-gewerbes und ihre wichtigsten Bestandteile. Bechhofen, Germany: Zentralverband der Bürsten-und Pinselhersteller.

Di Bernardo, D.J. 1945. Painting and Decorating. New York: Van Nostrand Company.

Harley, R.D. 1972. Artists' brushes–Historical evidence from the sixteenth to the nineteenth century. In Conservation of paintings and the graphic arts, Lisbon Congress, International Institute for Conservation of Historic and Artistic Works, 1972. London: International Institute for Conservation of Historic and Artistic Works:123–9.

Janovic/Plaza's Incomplete Catalogue for Decorative and Scenic Painters. 1990. Long Island City, N.Y.: Janovic/Plaza.

Mayer, R. 1982. The Artist's handbook of materials and techniques. 4th ed. New York: Viking Press.

Newell, A.C. 1930. Coloring, finishing and painting wood. Peoria, III.: The Manual Arts Press.

Painting and Decorating Contractors of America, Inc. 1965. Painting and decorating craftsman's manual and textbook. Chicago: Painting and Decorating Contractors of America.

Saitzyk, S.L. 1987. Art hardware: The Definitive guide to artists' materials. New York: Watson-Guptill.

Turner, J. 1992. Brushes: A Handbook for artists and artisans. New York: Design Press.

Veliz, Z., ed., 1986. Artists' techniques in Golden Age Spain: Six Treatises in Translation. London: Cambridge University Press.

Welther, L. 1991. Die Geschichte und die Herstellung des abendländischen Künstlerpinsels. Stuttgart: Akademie der Bildenden Künste. (Diploma thesis, Institut für Technologie der Malerei, Staatlichen Akademie der Bildenden Künste, Stuttgart.)

Spray Varnishing Equipment

Introduction

Spray guns were first developed in the 1890s for the application of whitewash in the construction industry, followed by usage in the agricultural community. With widespread access to air compressors (which were first produced in the mid-19th century), compressed air became the method of choice for the application of paints and lacquers in the automotive industry. Although a mention of its usage in conservation is cited by Helmut Ruhemann in 1931 (Ruhemann 1968, 277), spray equipment probably did not become readily available to conservators and restorers until the mid 1930s and 1940s. Spray varnishing seems to have become a common procedure for the application of surface coatings in the 1950s.

Spray systems are presently manufactured in the following formats: conventional high-pressure, low-volume (HPLV); conversion high-volume, low-pressure (HVLP); turbine HVLP; airless; and airbrushes.

[Ed. Note: The authors thought it best not to recommend specific brand names of equipment in this entry. There are so many different types available, there are constantly new products on the market, and individuals find different types suit them differently. Each conservator should test them and see which works best for his/her needs.]

High-Pressure, Low-Volume (HPLV)

a) Principle

The traditional spray equipment found in many conservation facilities consists of three main components: an air compressor (including a tank, regulator, and hoses); an oil and water filter; and a spray gun.

Conventional spray guns operate by using a low volume of air (minimally 2.5 cubic feet per minute [cfm], but usually 5–10 cfm) at high pressure (20–40 pounds per square inch [psi] or higher) to atomize the varnish solution. Atomization occurs when the liquid enters the air stream either in an internal-mix air cap, which mixes the varnish and air inside the gun, or by an external-mix air cap which atomizes the varnish and shapes the spray pattern outside the cap. The volume of air used and the spray pattern is determined by the size and placement of the holes in the air cap.

The varnish fluid is usually delivered into the air path from a container attached to the gun, although some systems permit the use of a remote cup or pot. These containers (cups) can be either gravity-feed, mounted on top of the gun; or siphon-feed, mounted below the gun, relying on a Venturi or a vacuum to draw the liquid from the cup into the gun. The cups are usually made of aluminum, stainless steel, plastic, or can have Teflon linings. Do not use unlined aluminum cups or guns with aluminum “wetted parts” (the internal components of the gun which come in contact with the liquid solution, such as the fluid passages, needles, and tips) with any alkaline water-based materials or chlorinated solvents as these will quickly corrode the aluminum.

Stainless steel wetted parts are preferable to mild/low carbon steel or brass components. Non-stainless steel parts can wear out easily or rust. The gun body itself may be made of stainless steel, aluminum, or even plastic. Although it is lightweight and less expensive, plastic may not be very durable.

Bleeder-type guns are used with continuous run compressors or with compressors that do not have air tanks. Air blows out of the gun at all times, varnish only being sprayed when the trigger is pulled. Non-bleeder guns are used with compressors with air tanks. The trigger controls both the air and liquid flow.

b) Compressor

The compressor is the key component in the spray system. Both the horsepower rating and the tank size affect spray performance. Internal-mix guns can be driven by a 1-hp or a 2-hp compressor, while an external-mix gun requires at least a 3-hp, with 4–5 hp being preferable. A 3-hp compressor with an output of 10 cfm and a 20-gallon air tank should be considered as a minimum.

Selection of the correct compressor is also dependent upon its rated duty cycle, the percentage of time per hour that the pump should be allowed to run. The higher the duty cycle, the longer the pump can run. The pump life of a compressor is affected by its duty cycle, the ambient operating temperature, maintenance, and air intake quality.

Compressors chill the air, condensing any water vapor present into a liquid, making removal of the water critical to the spray system. Oil used to lubricate the pump can also get into the air stream. An oil and water filter must be placed as close as possible to the spray gun to remove the contaminants. An air-pressure gauge and an air regulator modify the pressure coming into the gun.

c) Spray Patterns/Control

Pulling the trigger on the gun extracts the needle from the fluid tip, allowing the liquid varnish to enter the air stream. Fluid set-ups (the fluid tip and needle combination) are sold in matched sets. The size of the fluid set-up and air cap, along with the viscosity of the liquid to be sprayed, determines the amount of material delivered. The air pressure atomizes the liquid into droplets ranging in size from about 15 microns in diameter to 70 microns or larger, depending upon the viscosity of the material and the equipment used. Generally, the smaller the droplets, the better the varnish film formed.

Rotating the air cap on external-mix guns changes the orientation of the spray “fan” from vertical to horizontal. Controls on the gun can adjust the air to fluid ratio, producing a “wetter” or “dryer” spray. Distance from the target will affect the width of the fan and the distribution of the varnish droplets.

d) Maintenance

Compressors and moisture traps/filters must be drained or bled daily and immediately before spraying to avoid rust and blowing of liquid water into the varnish and onto the artwork. Filters should be replaced per the manufacturer's specifications.

A solvent (such as toluene, or of low polarity to avoid rust) should be sprayed to clear the fluid path of varnish after every use. Occasional removal and soaking of the fluid setup and air cap in solvent will maintain performance. Some conservators have their guns “overhauled” (repacking, replacement of worn parts) every three to five years to keep the gun in optimum working condition and to protect their investment.

e) Advantages and Disadvantages

(1) ADVANTAGES

Internal-mix: Requires only a small (1–2 hp) compressor.

External-mix: Versatile; many different fluid tip/needle combinations are available for a variety of applications. Produces a finely atomized, easily adjustable spray. Many accessories.

Compressor: Can be used for tasks other than spray varnishing.

(2) DISADVANTAGES

Internal-mix: Most guns produce a coarsely atomized spray, unsuitable for fast drying varnishes. Limited to applying slow drying solutions whose droplets have time to flow together for film formation. Fewer choices of fluid setups. Tends to clog.

External-mix: Requires a larger compressor (3–5 hp). Because they use lots of air, these guns produce a lot of overspray or bounce-back of the finely atomized droplets, creating materials waste and a health and environmental hazard (use of these guns may eventually be phased out due to environmental regulations).

All HPLV guns should be used with proper personal protective equipment and a correctly operating spray booth or exhaust system.

Note: Never spray towards unprotected skin as the highly pressurized air will force the finely atomized solvents and resin through the skin and directly into the bloodstream.

Compressor: Noisy, bulky, difficult to transport. Delivers cold/cool air to the gun under high pressure. Danger of water and/or oil contamination in the air line if the filters are not maintained or the unit is not regularly bled.

High-Volume, Low-Pressure (HVLP) Conversion Guns

a) Principle

First introduced in 1989, conversion-air HVLP systems convert high pressure air from a conventional air compressor to a high volume of air (50–100 cfm) at a low pressure (2–10 psi) by passing it through baffles and expansion chambers in the gun. This “soft” spray moves at a slower speed than high pressure systems and produces much less overspray or bounce-back, creating a cleaner and safer work environment, using and wasting less resin and solvent.

Transfer efficiency (TE) is the measurement of the amount of material actually deposited on the target as compared to the total amount sprayed. Conventional high pressure guns have a TE of 30–35%. Most manufacturers of HVLP systems claim a TE up to 80%. California SCAQMD (South Coast Air Quality Management District) regulation #1136 mandates that an HVLP spray system must have a gun air cap pressure of 10 psi or less and a TE of at least 65%. Most states have accepted this definition as the industry standard. Conversion to an HVLP system may be required in some areas by law (and may also require the use of high-solids, low volatile organic compound [VOC] materials when spraying).

The liquid is atomized at the gun tip much like an external-mix air cap on a conventional high pressure gun.

b) Compressor

Same as conventional system, 3–4 hp. Requires an oil-water filter (extractor/separator). Air pressure is adjusted by a regulator.

Conversion systems work best when connected to 3/8” air hoses so that more air is delivered to the gun to atomize thicker solutions. Quick-connect fittings on the hose and the gun should be matched—a 1/4” coupler negates the advantage of a 3/8” hose.

c) Spray Patterns/Control

All conversion guns provide air and fluid adjustments. The gun must be matched precisely with the material to be sprayed by controlling the air pressure and amount of fluid delivered at the gun tip. The air cap, fluid tip, and needle must also match the type of material sprayed. Separate air and fluid controls are available on some guns while others have a single knob for adjustment.

The width of the fan pattern can be controlled by either a knob on the gun body or by rotating a ring on the air cap. Line pressure is controlled by a regulator. The air, fluid, fan pattern, and line pressure must all be carefully adjusted. Because the spray comes out of the gun at a lower pressure, different resins/viscosities may atomize differently than one is accustomed to, perhaps forming larger and fewer droplets. Practice is required when changing from a conventional spray system to an HVLP. It may be necessary to hold the gun closer to the target and/or to use a slower hand speed.

HVLP guns generally come in siphon feed, pressure feed, or gravity feed models. Most conversion guns employ pressure feed systems, using some of the compressed air to pressurize the paint cup (usually a siphon feed at the bottom of the gun). Pressure feed guns normally require less air pressure to atomize the liquid. Caution must be used as an overpressurized cup may leak or even rupture. Screw-on cups tend to leak less than quick-release models but can be more difficult to change.

d) Maintenance

Same as for high pressure systems.

e) Advantages and Disadvantages

(1) ADVANTAGES

HVLP conversion guns produce much less overspray and bounce-back; they conform to “new” environmental regulations; can be used with an existing air compressor (as long as it is powerful enough); produce a finely atomized “soft” spray when properly adjusted; and the lower pressure displaces the painting less.

(2) DISADVANTAGES

HVLP conversion guns are more expensive than conventional guns; they require precise matching of fluid setup and controls to the material being sprayed; require practice and an adjustment period to get used to using an HVLP system, perhaps altering viscosity formulas to achieve the proper results. Pressurized cups could leak or rupture if overpressurized.

(3) COMPRESSOR

Same as for high pressure systems.

High-Volume, Low-Pressure (HVLP) Turbine Systems

a) Principle

Unlike conversion-air HVLP systems which are driven by a conventional high pressure air compressor, turbine HVLP systems employ a fan to create a high volume of air (45–110 cfm) at low pressure (2–7 psi). The turbine blows a constant stream of warm, dry air at a consistent pressure. The warm air can improve varnish flow and film formation.

The guns look like and operate in a similar manner to conventional and conversion guns. They incorporate larger air passages to deliver higher volumes of air to the air cap for fine atomization. Because the air flow is warm (or hot, up to 140°F), the handles of some guns can become too hot to grip unless some method of insulation or heat dissipation is incorporated into the design.

Guns are manufactured in bleeder and non-bleeder styles. A bleeder gun allows air to bypass the cup and blow out through the nozzle when the trigger is released to prevent overheating of the turbine. This constant airflow can blow dust or debris onto a freshly varnished surface if care is not taken. Non-bleeder guns have a bleed off valve mechanism sometimes mounted on the turbine which can restrict airflow. Use of a bleeder gun can extend the life of a turbine.

Most HVLP guns come with pressurized aluminum cups. Purchase a Teflon® lined cup or a stainless steel cup if alkaline water-based solutions or chlorinated materials are to be used. Stainless steel wetted parts are preferable to other types of component materials which could wear out quickly, rust, or corrode.

Turbines operate much like a vacuum cleaner and are compact and easily portable, making them attractive for use in small spaces. Some can be used as a backpack, particularly useful for large-scale projects.

Like conversion HVLP systems, turbine HVLPs produce much less overspray and bounce-back than conventional high pressure systems, for a cleaner, safer environment, less material waste, and conformance with environmental regulations.

b) Turbine

The components consist of a high speed electric motor, one or more fans or blowers, and one or more air filters in a plastic, aluminum, or steel housing.

Each fan is referred to as a “stage”; a “three-stage” turbine has three fans driven by one motor. The number of stages, the size of the fans, and the motor output determine the air pressure and volume. Small one- or two-stage units are adequate for spraying low viscosity solutions or small projects. Larger jobs or thicker materials may require more powerful turbines (each stage adds approximately 40 cfm and 2 psi to the air output). Either a large two-stage (i.e. two 7.2” diameter fans) or a small three-stage (i.e., three 5.7” diameter fans) should be powerful enough for any type of job. The higher the psi and cfm ratings, the more powerful the unit.

One or more air filters supply the turbine with clean, dust-free air. These filters should be able to be removed for cleaning several times before needing replacement. Keeping them clean will improve the turbine's efficiency and prolong the life of the motor. The turbine should be kept as far away from the gun as possible during spraying to avoid clogging the filter with any spray mist.

Turbines use larger air hoses than conventional air compressors, typically 1” outer dimension with a 5/8” or 3/4” inner dimension. Their bulk can make the gun hard to maneuver unless the gun end of the hose has a flexible extension (“whip”).

Because the air is clean, warm, and dry (unlike conventional compressed air), there is no need for pressure regulators or filters to remove oil or water from the line.

c) Spray Patterns/Control

Most HVLP guns can adjust the air flow, fluid flow, spray width, and spray patterns, the method of adjustment varying with the manufacturer. As with conversion HVLP guns, it is necessary to match the needle size/fluid viscosity with the settings on the gun for proper atomization. Practice with this type of system is even more critical than with a conversion gun due to the warm air, which produces different spraying characteristics from those of a conventional high pressure system.

The fluid tip and needle combined with a matching air cap is known as the nozzle assembly or “projector set.” Smaller needles produce a finer spray when used with low viscosity solutions. Larger sets are needed for high viscosity materials or to speed up the spraying of thin materials. Rotating the air cap alters the spray pattern from vertical to horizontal or round, similar to a conventional external-mix gun.

d) Maintenance

Care must be taken to clear the fluid path with solvent following spray varnishing to avoid buildup of resin on the fluid tip due to quick drying caused by the heated air.

Turbine air filters must be kept clean or be replaced to protect the turbine from overheating and to avoid dirt and debris from entering the air stream.

e) Advantages and Disadvantages

(1) ADVANTAGES

Gun: Warm, dry, clean air can produce a better coating than the cool, moist (if not properly filtered) air from a conventional high pressure or a conversion air HVLP system. “Soft,” finely atomized mist spray produces less overspray and bounce-back for a cleaner, safer workspace and uses less resin and solvent. Conforms to “new” environmental regulations.

Turbine: Usually less noisy than a conventional air compressor. Small, lightweight, and completely portable. Some models can be used as a backpack. Does not require regulators or oil/water extractors. No need to purchase a compressor if one is not required for other tasks.

(2) DISADVANTAGES

Much more expensive than a conventional high pressure or a conversion air system. (Note: it is possible to purchase a turbine driven gun which can be adapted for use with an air compressor by connecting a low-pressure regulator or “air amplifier” between the hose and the gun inlet. This could be a good choice if you already have a compressor but are considering purchasing a turbine in the future.)

Gun: Some handles can become uncomfortably hot if used for long periods of time. If not regularly cleaned, dried resin can accumulate at the nozzle tip, affecting spray performance or breaking off and being deposited onto a freshly varnished surface. Bleeder guns can blow dust or debris onto the surface if care is not taken.

Turbine: Unlike an air compressor, it cannot be used for other tasks. Air filters must be kept clean to avoid overheating. For proper atomization for viscous materials, the turbine must produce enough cfm or a coarse spray might result.

Airless Sprayers

a) Principle

Airless sprayers are designed mainly to spray heavier coatings than conventional or HVLP sprayers, usually for industrial or painting contractor use. They are lightweight, convenient, and faster to use than other types of spray systems, allowing the use of viscous materials and providing greater coverage per pass. They generally produce a coarser spray than other types of sprayers.

The gun is usually made of high density polymers, lightweight and solvent resistant. The motor housing should keep out solvents and combustible vapors. The cups are usually polyethylene or nylon with options for attachment to a remote container or backpack container for holding larger amounts of liquid.

b) Pump

Instead of using an air compressor or a turbine, airless systems employ an electric pump to force the liquid out of the nozzle at high pressures (2,500 psi or higher). “Power Painters” use a motor armature to drive a piston, creating a vacuum which pulls the liquid up from the cup, then pressurizes it out through the tip of the gun. Diaphragm pumps siphon the liquid into the gun by using a high speed motor to move a diaphragm back and forth to create a pumping action. Piston pumps are dual acting, creating pressure on both the up and down stroke. Diaphragm and piston pumps are prohibitively expensive and are intended mainly for industrial or heavy-duty use.

“Power Painters” are the familiar models designed for home use. The pumps are usually a hardened steel piston with a tungsten carbide cylinder and a chrome steel ball valve. Motor size determines a gun's versatility and price. A 45-watt unit is suitable only for very thin liquids. A 110-watt gun will spray viscous materials. A motor rating of 85 watts or higher should be adequate for most uses. Except for backpack models, there are no hoses needed as the gun, motor, pump, and cup are usually a single unit which plugs into an electrical extension cord.

c) Spray Pattern/Control

Airless guns have electronic pattern controls to alter the width of the spray from narrow to wide. A variety of tips or nozzles are available for spraying of different materials. Matching the fluid viscosity to the gun and the proper nozzle or tip is crucial for peak performance. Viscosity testers are supplied with the guns. Incorrect viscosity will result in a poorly atomized, coarse, and blotchy surface. Airless spray coatings are usually more coarse than other types of systems. Slow drying materials are more appropriate to use with airless systems than fast drying solutions.

d) Maintenance

The gun, especially the nozzle/tip, must be kept clean to avoid “spitting.”

e) Advantages and Disadvantages

(1) ADVANTAGES

Inexpensive, lightweight, self-contained, easily portable. Designed to spray a wide range of materials, especially heavy viscosity. Wand extensions and other accessories are available for spraying in tight spaces or difficult conditions.

(2) DISADVANTAGES

Airless sprayers generally produce a less fine coating than other types of systems and can be extremely coarse and blotchy if the fluid viscosity is incorrect. Although this type of gun is really designed for application of industrial and painting contractor finishes and coatings rather than for delicate spray varnishing of artworks, it might find use in a conservator's repertoire given enough experimentation and practice.

Airbrushes

a) Principle

Airbrushes work on the same principle as that of high pressure-low volume spray guns. Designed for arts and crafts applications, airbrushes offer more precise and smaller scale application. Their usage in painting conservation includes local varnishing of retouching, overall varnishing of small-scale works, and reintegration of large amounts of loss or abrasion, especially in mural projects.

Equipment includes the airbrush, a fluid containment system, and an air delivery system. The airbrush may have a selection of different needles and heads (also known as nozzles or cones) to allow variation in the size and shape of the spray. Needle and head assemblies are interchangeable for some airbrushes, allowing variation in fineness of application. The needle is adjusted within the cone to partially block the opening thereby changing the amount of air or air/fluid exiting the airbrush.

b) Air Source

(1) CANNED AIR

Pressurized air sold at hobby stores. Good for on site work, but can be costly and difficult to regulate the pressure.

(2) COMPRESSED AIR

Compressors currently in use for spray guns can be fitted for use with airbrushes by adding adapter couplings for different size hoses. Airbrushes run well on 0.5 to 1.0 c.f.m. and 20–45 psi depending on the viscosity of the liquid being sprayed. Moisture traps and pressure regulators currently installed on compressors are sufficient. Small compressors designed for airbrushes are available, but do not have a large enough tank or produce enough c.f.m. for use with full size HPLV or HVLP conversion spray guns. A small portable compressor for on-site work may be desirable.

c) Airbrush Types/Control

Types of options for airbrushes are similar to those for spray guns.

(1) FLUID CONTROL

(a) Single Action

Fluid is applied by pulling the trigger, and exits the airbrush at a pre-set rate. This can be changed by a valve, but not while being used.

(b) Double Action

The amount of fluid which exits the airbrush depends on how far the trigger is pulled. This allows more variation in application.

(2) FLUID/AIR MIX

(a) Internal Mix

The fluid and air mix before leaving the airbrush. Lower viscosity liquids are more appropriate for this type of gun as the heads can tend to clog.

(b) External mix

The fluid and air mix after leaving the airbrush, allowing usage of higher viscosity liquids. This tends to give a coarser spray than the internal mix type. External mix airbrushes also require higher pressure.

(3) FLUID DELIVERY

(a) Siphon Feed

The fluid from a cup or jar on the bottom or side is delivered to the gun air stream suction. One drawback is that the siphon tube is usually plastic, and may not be able to be used with some solvents.

(b) Gravity Feed

The fluid is delivered directly to the air stream through gravity from a metal cup or cavity on top of the airbrush.

Note: Most airbrushes have been designed for the graphic artist using dilute aqueous mediums such as watercolor or ink. Conservators often use higher viscosity varnishes and color mediums which can cause clogging and spattering. The larger nozzles more suitable for these materials produce a heavier, coarser spray than those intended for use with water-based media, with higher pressure required and external-mix preferred. Only one external-mix double-action airbrush is currently available, and may be more suitable for very fine detailing. If area coverage is needed, then a single-action external-mix would be preferred for higher viscosity materials. Because of the tendency for airbrushes to clog, an airbrush which is easy to clean is desirable.

d) Maintenance

(1) CLEANING

Due to the nature of the materials that conservators may use in an airbrush, cleaning becomes very important to ensure proper functioning. For most internal mix models, this means dismantling the airbrush and then soaking the parts in an appropriate solvent. The manufacturer's information should be consulted in order to determine a proper maintenance schedule. Certain models of airbrushes have “easy-care” maintenance because of the design. The air mix may be confined to the nozzle so that the entire airbrush does not need to be dismantled. When purchasing an airbrush, it is best to compare their features and the ease of cleaning.

(2) MOISTURE

Bleeding of the compressor and moisture trap should be done before using the airbrush.

(3) PARTS

Airbrushes contain many small parts including the needle, head, washers, and siphon tubes. The needles of an airbrush are susceptible to bending, which prohibits proper functioning. The needle and head assembly are sold as a set, and cannot be interchanged with mismatched sizes. Extra needles can usually be purchased separately in case of damage or loss. If several airbrushes are in use, it is best to keep all parts to one brush together so that complete assembly can be accomplished.

e) Health and Safety

Health and safety concerns are the same as that for spray guns. Atomization of toxic solvents simplifies the potential for inhalation. Spraying should be executed in a properly ventilated area.

f) Advantages and Disadvantages

The single-action, external mix airbrushes are the simplest and most economical, but are limited in application due to less variance of spray size and shape. A double action airbrush would give more variation, especially for the applications used in conservation, but since most double-action airbrushes are internal-mix there are fewer viscosity options.

Airbrushes can be used with an existing spray gun compressor with the proper adaptor or with canned air if the purchase of a small-size compressor is not being considered.

References

Apollo Sprayers. 1995. Apollo Spray Air Conversion HVLP. True HVLP from your compressor. http://orion.adnc.com/~websites/apollo/conversion.html. 20 December 1995.

Apollo Sprayers. 1995. How to select the right HVLP spray system. http://orion.adnc.com/~websites/apollo/selecting_a_system_html. 20 December 1995.

Barrett, J. 1995. HVLP conversion spray guns. Woodworker's journal 19(6):68–74.

Barrett, J. 1996. HVLPs:Turbine spraying systems. Woodworker's journal 20(1):84–9.

Bernstein, J. 1992. A Review of varnish application fundamentals. In 1992 AIC Paintings Specialty Group Postprints, Buffalo. Washington, D.C.:American Institute for Conservation of Historic and Artistic Works: 111–19 (Note: An excellent glossary of common varnishing problems and solutions, pp. 117–18).

Binks Manufacturing Company. 1990. 1890–1990: One Hundred Years. [n.p.]

Bradley, M.C., Jr. 1950. The Treatment of pictures. Cambridge, Mass.: Cosmos Art Technology.

Blick, D. 1996. Airbrush comparison chart. [1]. 21 February 1996.

Blick, D. 1996. Airbrush questions and answers. [2]. 21 February 1996.

Blick, D. 1996. Dick Blick Art Materials Catalogue. [Dick Blick Art Materials, P.O. Box 1267, Galesburg, Il 61402–1267.]

Grainger, W.W. 1996. Catalog No. 387. [n.p.]

Jerry's Artarama. 1995. Catalog No. 40 B. [Jerry's Artarama, P.O. Box 1105, New Hyde Park, NY 11040.]

Liberon/Star Supplies. 1995. Trouble shooting lacquer finishing problems. [3]. html. 11 December 1995.

Minick, C.A. 1995. Which spray system is right for you? Taunton's Fine Woodworking (August): 58–61.

Ruhemann, H. 1968. The Cleaning of paintings: Problems and potentialities. London: Faber and Faber; New York: Frederick A. Praeger: 277–8.

Wehlte, K. 1982. The Materials and techniques of painting. 3d ed. New York: Van Nostrand Reinhold:542–3.

Use of the Mouth Atomizer to Varnish Paintings

Purpose

The mouth atomizer, also known as the blow atomizer, can be used effectively for the spray application of varnish but is a tool best adapted to producing a moderately heavy, saturating film.

Factors to Consider

The atomizer performs essentially the same function as a mechanically powered spray system but does not afford the flexibility that powered systems have to achieve a wide range of surfaces by the adjustment of a number of variables. When properly used, this tool can produce a varnish surface that is well suited to the aesthetic needs of many traditional oil paintings. However, it is not usually appropriate for varnishing paintings that have very smooth surfaces because in such cases a detectable spray pattern will become evident.

This same tendency to produce a “false” surface can be used to advantage to integrate glossy spots and to unify paintings that have required selective cleaning. In extreme cases it can be used to somewhat “animate” a paint surface that has been severely damaged by improper lining.

Materials and Equipment

The atomizer consists of two open-ended metal tubes of differing lengths and diameters that are set at right angles to one another with their ends in contact. The longer and narrower of the tubes which goes into the liquid to be sprayed, is approximately 5” long and 1/16” diameter. The shorter air tube is approximately 3” long and 1/4” diameter and sometimes has a slight taper. The user blows air through the shorter, larger diameter tube which causes the liquid to be siphoned up the narrow tube. The stream of air then disperses the varnish into fine droplets and carries the mist to the surface of the painting. (Gettens and Stout 1966).

The air pressure must be forceful and steady for the varnish to be properly atomized. Otherwise, the droplets will be too large and irregular which will spoil the surface. Thus, unlike mechanically powered spray systems, varying the air flow is not an option for controlling the appearance of the varnish.

In addition, the varnish solution, if it is too concentrated, will not be properly dispersed and will produce a pebbled surface. The best varnish consistency for spraying is that of a retouching varnish. Proper consistency allows the varnish to be broken into fine droplets which will tend to merge into a continuous film when they touch the surface of the painting.

Treatment Variations

Varnish can be applied with the painting standing vertically but this can make it difficult to achieve an even distribution. It tends to cause overvarnishing in spots and has the potential of causing drips in the surface.

The best arrangement is to have the painting lying face up on a waist-high table or on sawhorses. One takes a position to the side of the painting and from that position can varnish an area of approximately 3 x 3 feet using successive side to side sweeps that cover overlapping bands of approximately 1 x 3 feet. Ventilation should be set up with a flow of air coming from behind the person and across the surface of the painting. This is essential to avoid inhaling the spray that is being generated. The varnish solution should half fill a glass container which is about two inches in diameter and about four inches deep. This will protect against accidental spills.

The first sweep should be made at the farthest distance that the person can reach by bending 45° forward from the waist. For a person of average height, the spray should reach approximately three feet into the painting. One then takes a step back, and proceeds with an overlapping pass in the other direction. It should take four or five sweeps to cover an area 3 x 3 feet.

The atomizer is held in a range of about 10 to 15 inches above the surface of the painting and is angled slightly downward. The distance and angle are the main variables that can be adjusted to alter the distribution of the varnish. A spray which is dispersed from a close range and which is angled toward the surface of the painting will produce a very heavy and saturated coat. A spray from a higher range which is angled almost parallel to the plane of the picture will produce a somewhat drier surface because the droplets will tend not to merge completely.

A varnish coating may be built up in layers using this method. However, a minimum of 24 hours of drying time should be allowed between succeeding varnish applications in order to avoid problems.

References

Gettens, R.J. and G.L. Stout. 1966. Painting materials: A short encyclopedia. Unabridged and corrected 1942 ed. New York: Dover Publications.

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