Varnishes and Surface Coatings: General Application Techniques

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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.

Author: Wendy H. Samet
Date: Submitted January, 1997
Compiler: Wendy Samet

Purpose

The purpose of this section is to review techniques used for the application of surface coatings either as isolating layers or as finishing layers on paintings, to achieve the degree of saturation or gloss desired.

To repeat the disclaimer from the Chapter Introduction, pages xi-xiv, this information is designed for the practicing painting conservator and is not intended to establish definitive procedures nor to provide step-by-step recipes for the untrained. The individual conservator is solely responsible for determining the safety and adequacy of a procedure. Inclusion in this entry does not constitute an endorsement or approval of the procedure or material described. Many of the procedures are described in anecdotal and incomplete form.

Factors to Consider

Optimum Conditions for Varnish Application

Varnishing is best accomplished in controlled museum or studio conditions of moderate and stable relative humidity and with excellent fume extraction. Varnishing is sometimes done outside. A mild day with approximately 50% relative humidity is preferable. Winds should be very low and the painting should not be in direct sunlight.

Timing

Cleaning should be complete to the extent desired and all cleaning solutions should be allowed to evaporate for at least a week, preferably longer, before the first coat of varnish is applied. Adequate time should be allowed to elapse between varnishing sessions.

Optimum Amount of Varnish to Be Applied

It was the consensus of all practitioners interviewed that it was best to apply as little varnish as possible to achieve any given appearance.

Preparing Resin Solutions (Whitten 1997)

Resin Solutions

As a general rule, it is best to use fresh resins and solvents for varnish mixtures. Resins and solvents degrade in storage. They degrade even more quickly in solution so it is a good practice to mix small batches and discard the unused portions periodically. Avoid stock solutions unless they will be used quickly. A word of caution about stabilizing additives: Mixtures with HALS or UV absorbers should be discarded after one month because these stabilizers may be consumed in solution. The stabilizing reaction of Tinuvin® 292 continues catalytically in a dried resin film. Although we have no way of knowing how long conservation supplies have been stored prior to being purchased, it is a good practice to label and date new materials as they enter your lab or studio.

Varnish Mixtures

Varnishes can be mixed and measured in any number of ways. It does not matter what method is used, but confusion can arise when recipes are described as “percentages.” In the scientific field, the following methods are used for weight to weight, and weight to volume percentages.

a) The resin and solvent are both weighed. This is described as weight to weight (w/w).

25% solution:

25 g of resin

75 g of solvent

100 g of varnish

b) The weighed resin is placed in a volumetric flask and the solvent is added up to the 100 ml line. The total volume is 100 ml. This is called weight to volume (w/v). You will not know the exact amount of solvent in this mixture.

Letters a) and b) take the density of the solvent into account. They are described by Suzanne Quillen Lomax in a 1995 AIC News article (Lomax 1995).

Two other methods are often seen in the conservation field. These are other types of w/v mixtures that cannot be described as percentages.

c) A mixture is made by weighing the resin and measuring the milliliters of solvent. The numbers will add up to 100.

25 g of resin

75 ml of solvent

d) The resin is weighed and 100 milliliters of solvent are added.

25 g of resin

100 ml of solvent

Percentages can be calculated for each of the three w/v methods above but you must know the density of the solvent to calculate the last two. For our purposes, each method is a perfectly acceptable way of working. The confusion arises when a particular concentration is trying to be conveyed and the word “percentage” is used. The actual ratio of resin to solvent is different in each of these four methods. Method d) has been used for the Regalrez® 1094 research because it was important to keep one amount constant (100 ml of solvent) for clear comparisons.

Measuring Tinuvin® 292

Tinuvin® 292, a hindered amine light stabilizer (HALS), is added to varnishes to slow degradation. The stabilizer is added as a percentage of the weight of the resin: the amount of solvent will not affect the amount of Tinuvin® 292 and should be added to achieve the desired working properties. For Regalrez® 1094 and MS2A®, it is added at 2%. For inherently unstable resins such as damar or mastic, it is added at 3%. Tinuvin® 292, and resins in general, are less stable in solution than they are in a dried film. So, mixtures should be made in small batches which, when Tinuvin® 292 is added, should be discarded after one month.

As an example, we will make a varnish mixture containing 25 grams of resin:

For Regalrez® 1094, the Tinuvin® 292 is added at 2% of the weight of the resin:

Regalrez® 1094		25 g
multiply by 2%		x .02
		0.5

There are several ways to measure the 0.5 g of Tinuvin® 292:

a) with a scale that is accurate to 0.00 (two decimal places—the scale should be one decimal point more accurate than the amount you are weighing); or

b) With an accurate pipette or syringe;

Tinuvin® 292 has a density similar to water
H2O	= 1
Tinuvin® 292	= 0.992

Therefore, for Tinuvin® 292 or water

1 g = 1 cc - syringe

1 g = 1 ml - pipette

c) With an accurate scale, determine how many drops are equivalent to 1.0 g. Then you can always use the same pipette and add the Tinuvin® 292 dropwise.

Example: for pipette “X” there are 27 drops in a gram, so if you need 0.5 g

27 drops per gram

x 0.5

13.5 add fourteen drops of Tinuvin® 292.

Or

d) This fourth method requires that you weigh the solvent and the resin (w/w). If no balance for weighing small quantities is available, Tinuvin® 292 can be dissolved first as a stock solution at a concentration of 10% in the solvent you are using for the varnish. You will multiply the amount of Tinuvin® 292 you need by 10 to calculate the amount of stock solution. For 25 grams you would need 0.5 grams Tinuvin® 292. You would then add 5.0 grams of the Tinuvin® 292 stock solution to the resin and the amount of solvent should be reduced accordingly:

Recipe:

Regalrez® 1094		25.0 g
Tinuvin® 292 (2%)		0.5 g
Solvent		74.5 g
		100.0 g

Stock solution calculation:

Tinuvin® 292 you need 0.5 g
multiply by 10	x 10
	5.0 g

Tinuvin® 292 stock solution:

Tinuvin® 292		10.0 g
Solvent		90.0 g
		100.0 g

Varnish:

Regalrez® 1094		25.0 g
Tinuvin® 292 stock sol		5.0 g
Solvent		70.0 g
		100.0 g

Any remaining stock solution must be discarded after six months.

Methods of Brush Application

Common Uses

Brushed coats of varnish are usually used for an initial, saturating, and isolating layer directly on top of the paint layer. Application of varnish by brush may also be used in later layers, provided that intervening inpainting and lower varnish layers are not soluble in the solvents of the varnish being brushed. However, some practitioners have developed techniques which allow them to brush varnishes over lower layers of varnish and inpainting with the same solubilities. See Individual Practices and Tricks of the Trade, Section H. below.

General Application Procedures

Varnish of the correct dilution should be placed in a wide-mouthed jar that is at least an inch wider than the varnish brush. A vessel of low profile and some weight is best in that it is unlikely to tip over. A small amount of varnish is placed in the bottom of the jar. The lower third of the brush is dipped in the varnish and excess is removed by wiping and draining against the sides of the jar. Some conservators eliminate excess varnish from the brush by wiping it briefly on a lint-free, absorbent cloth or paper towel (Bernstein 1992, 111–19).

It is important to estimate the amount of coverage possible with each dip into the varnish. Although there are many approaches to brushing out the varnish as noted below, in general the loaded brush is placed in the center of the section to be covered. The solution is then spread out until evenly saturating the entire section. Then the next dip is placed the center of the next area to be covered, not directly adjacent to the previously varnished area. When sufficient brushing time is possible, the varnishing may be finished with continuous light strokes over the entire area of the painting, using the same varnish brush without dipping again into the varnish solution, or finishing with a clean, thin-bristle brush. Bernstein suggests finishing with strokes beginning from the bottom of the canvas, moving to the top. This can help to fill in the undersides of the weave or impasto reducing banding, shadows, and specular reflection (since paintings are mostly lit from above.)

The Orientation of the Painting

The painting may be placed in one of three orientations each with its own advantages and uses:

a) Horizontal

This is the usual orientation of a painting for brush varnishing. The painting lies face up on a table with good light and adequate ventilation. The likelihood of drips is mostly mitigated by this orientation of the painting during varnishing.

b) Vertical

Some conservators prefer to varnish a painting in the vertical position whenever possible as it allows for viewing the painting during varnishing in the orientation in which it will be seen. Others varnish in the vertical position primarily because it is the only orientation possible for some murals or larger paintings. Drips may be a problem. Varnishing should begin at the top edge and any drips brushed out as soon as they are observed. A glancing light will aid in viewing the varnish as it is applied and in detecting any irregularities so that they may be corrected.

This vertical technique is described more fully by Mark Leonard, Getty Museum, as follows:

The painting is placed on an easel in preparation for varnishing. This is done for two reasons. As with cleaning, which should also be done on an easel, it is important to see the picture during the process, which is simply not possible if it is horizontal. The second reason is that a vertical position naturally guards against the possibility of excess varnish building up on the surface. A very thin brush coat is always more successful than a thick one, and keeping the painting in a vertical position ensures that a minimum of varnish is used.
The varnish is poured from the mixing jar into a flat aluminum pan. When a synthetic resin is used, the varnish can be made immediately before use. When mastic or damar are used, and Tinuvin® 292 is added, the stabilizer is added to a small amount of varnish taken from a stock solution several days in advance, so that any trace of cloudiness can settle and clear. The varnish solutions should be very low in concentration (7% w/w for mastic; 25% for low molecular weight synthetic resins such as Regalrez®). (Ed. note: The actual recipes are - 10 g mastic in 100 g solvent, and 25 g Regalrez® 1094 in 100 g solvent.)
Use of a flat pan allows for pressing the brush nearly dry against the edges prior to application of the resin. For a small painting, a two- to three-inch-wide brush can be used. Varnish application begins in the middle and the painting can often be varnished with only one brush load of solution. Because extremely slow-evaporating solvents (such as mineral spirits or turpentine) are used, the surface must be brushed continuously for a number of minutes. (Use of fast-evaporating solvents, such as xylene, or high-molecular-weight resins, such as B-72, will not allow for continuous brushing.) If the brush has been properly loaded, and if brushing is vigorous enough, dripping is simply not a problem.
For larger paintings, a larger brush (up to five inches wide) is used. Varnish is applied in a “backwards L” pattern, beginning in the upper left corner. The backward L is applied in two strokes (each up to two feet long), the first in the vertical direction and the second in the horizontal direction; the remaining square is filled in from the bottom right to the upper left corner of the area. The next “backward L” is applied directly below, and the remaining square is once again filled in, this time brushing back into the previously applied area. This technique ensures that the first strokes of the brush (with the heaviest amount of varnish solution) are, for the most part, applied relatively far away from the previously varnished area, thus eliminating the danger of excessive build-up in overlapping areas. When varnishing a very large painting, it is very helpful to have a second person following along to continue vigorous brushing of the varnished areas. Once the entire painting has been varnished, and the surface has been continuously brushed until it has become slightly tacky, both people can finish the process by brushing over the entire painting with dry badger hair brushes in order to pull any excess residues off the surface.
Mastic has the advantage that it can be applied repeatedly in thin brush coats (as long as sufficient drying time is allowed between coats—up to four weeks is preferable). This can prove to be useful with pictures that have very uneven surfaces; repeated application of the mastic in vigorous brush coats tends to saturate dry, absorbent areas without causing smooth or glossy areas to become even more slick in appearance (Leonard 1996).

c) Tilted

This orientation of the painting may aid in determining the gloss by getting a glancing light on the work of art.

Applying the Varnish

Using a brush, the varnish can be applied to the horizontal painting in a number of ways.

a) Squares

In this application method the brush is dipped in the varnish and pressed against the side of the container to drain excess. The conservator works in small squares with strokes that run perpendicular to each other and are feathered at the edges. The conservator then moves on to the next area brushing in a similar pattern and joining the feathered edges of the adjacent squares until varnishing is complete.

b) Union Jack

The brush is fairly heavily loaded with varnish. Stripes are quickly applied from center top to bottom, from center right to left, and along each diagonal. The varnish is then spread to cover the remaining triangles of paint. This is often used as a strategy on fairly large paintings.

c) Stripes

The brush is loaded and parallel strokes are used, up and down, to cover the painting from top to bottom. Once one stripe is complete the next one adjoins it with some reworking at the leading edge.

d) Daubs

Mark Bockrath, David Bull - A brush, fairly heavily loaded with varnish is used to quickly make daubs or small pools of varnish scattered over the surface of the painting. These are then brushed out to create a uniform surface.

e) Center loading

A heavily loaded brush is used to apply varnish in the center of the painting which is then quickly brushed out to the edges.

f) Edge loading

A heavily loaded brush is used to apply varnish at the edges of a painting which is then worked into center.

g) Contoured

The varnish is applied along the contours of compositional features, especially when there are noticeable differences of absorption of varnish between one area and another.

h) Combination Center and Edge Loading

Sheldon Keck, as reported by Barbara Buckley: First apply a stripe of varnish along the two vertical edges of the painting. These stripes of varnish are applied with a loaded brush applied centrally and worked up and down. The varnish is then applied horizontally by applying a loaded brush centrally at the top edge of the painting and working towards the two side edges in a back and forth motion while keeping the edges feathered. The remaining horizontal brush strokes are applied slightly below the previous band of varnish as above and blended by feathering the edges. This method works best for medium sized easel paintings. The two vertical bands of varnish are usually deleted for small easel paintings.

i) Small Arcs in a Line

This method is described in an article by Philip Robinson in The Picture Restorer wherein he describes how to brush a final coat of a small molecule resin, (e.g., damar, mastic, or a ketone resin) over another such resin and retouching soluble in the same solvents. The article is more fully quoted below under “Individual Practices and Tricks of the Trade” (Section H.). Using a fairly large (50 mm x 22 mm x 57 mm) brush with springy, powerful, high-quality white bristles, the brush is dipped into the varnish and the excess removed first on the edge of the varnish pot and then by squeezing in a clean cloth. Starting at the far right and keeping the wrist rigid, the strokes are made by partially rotating the forearm, working from right to left. The brushing should be done firmly but without undue pressure. When the other side of the picture is reached, start again on the right hand side overlapping only just enough to blend the varnish into an even layer (Robinson 1995, 14–15).

Methods of Spray Application

(Bernstein 1992, 111–19)

Common Uses

Spray application of varnish enables great control of the quantity, gloss, and uniformity of the resin deposited. A major benefit of spray varnishes is that they may be applied without direct contact with, and physical disruption of, the painting surface. This is crucial for paintings with recently applied (or in-progress) retouchings that would be altered or removed with the action of brush varnishing. For contemporary paintings, spray application is also used for the uniform introduction of extremely dilute resin solutions, where the “varnish” serves not only as a surface modifier but primarily as a thin protective fixative or consolidant in an otherwise open and “unvarnished”-looking paint layer.

Note: Many conservators favor a hybrid technique. A preliminary saturating coat of varnish is applied by brush, followed by successive, thinly built-up finishing spray applications of varnish.

Principles of Sprayed Varnish Application

Most spray systems depend upon compressed or accelerated flow of air to draw, atomize, and disperse a varnish solution. The type of spray system and equipment has a major effect upon the results. The most elemental is the atomizer, using either lung or hand-pump air pressure and Bernoulli suction to draw a solution up a tube and disperse it into a spray. This method has largely been abandoned due to superior systems of atomization available. Small aerosol spray packs, powered by disposable propellant cans, also operate by external mixing and can be convenient for small jobs or on-site work. However, the atomizing mechanism remains primitive (the coarsely sprayed varnish may require brushing out) and some of the propellant compounds used for pressurization present environmental biohazards.

Electric compressors compress air to elevated pressures and are rated for the cubic feet per minute (or cfm). They can deliver at a given pressure per square inch (or psi). The compressed air is frequently forwarded to an air reservoir tank, from which an even flow of pressurized air is possible for a sustained spraying period. In the process of being compressed, the air is chilled and the relative humidity level increases dramatically. A moisture and oil filter trap, an air pressure gauge, and an air regulator are placed on-line to modify the air before it reaches the spray apparatus and artwork.

Airbrush sprayers require low volumes of air flow (1 to 2.5 cfm) at pressures of 15 to 30 psi, and area suitable to small-scale or detail work. Due to the fineness of the spray head and needle assemblies, airbrushes enable very fine atomizing of low volumes of varnish solution and great control of the build-up and specific areas being covered. While ideal for localized toning or building up of varnish, they are not practical for large pictures where banding may result.

For most painting spray applications, an industrial spray gun provides the capacity and capability needed for a variety of varnish techniques. Spray guns require a minimum of 2.5 cfm to typically 4.0 cfm or higher; and a working pressure of 20–40 psi. The gun and needle head housing may be small, medium, or large and it is beneficial to have more than one spray head assembly to suit the requirements of different treatments. In addition to the size of the spray head, fineness of atomization and pattern of spray options, the type of air mix and type of solution feed vary tremendously.

Because of frustrations with traditional air compressor-spray gun equipment that had been either inadequate, very lavish but not appropriate for their purposes, not understood or properly maintained, or the scene of many an unhappy coatings disaster, a number of conservators have switched to and prefer the Chiron® SC90E blower and PN2® Spray Gun (available from American Distributors, see Appendix II, Vendor Directory) or a similar type unit (available from W.W. Grainger Inc., locations nationwide), which uses a low-pressure, high-volume air flow, warming the air before mixing it with the varnish. Warm air spraying reduces ambient moisture and improves film-forming properties, and the low pressure will generally produce less over-spray. The lightweight air pump pack (working like a canister vacuum, only in reverse) can be worn on the operator's back, handy for large paintings where one has to cover a lot of territory.

Use of the Binks spray gun system allows for adjustment of the volume and nozzle aperture size with great sensitivity. The gun is connected to a separate compressor. In a Chiron® system the volume can be varied by trigger, but the aperture of the nozzle is changed only by selection of one of three interchangeable nozzles of different sizes. The Chiron® gun is attached to its own lightweight, portable, blower making it ideal for mural work. After several minutes of use, the Chiron® gun heats up the varnish somewhat. Since less sensitive adjustment of the spray parameters is possible for a Chiron® than a Binks gun, it is slightly harder to achieve the desired effect in spraying, especially when one tries to achieve a matte sheen (Bockrath).

A considerable amount of blending of local spray applications can be achieved by pulsing the spray with gentle pressure and release on the spray gun trigger or by spraying at a slight angle to the painting. Greater accuracy and control can be achieved by spraying through specially shaped openings in a piece of cardboard (not unlike dodging and burning in printing a photograph), gradually building up multiple layers of the varnish until the desired effect is achieved.

General Application Procedures

(Bernstein 1992, 111–19)

In general, solutions of varnish for spraying are more dilute than those used for brush coats.

Always pretest spray settings and solutions on scraps of board or discarded X-ray film sheet, to observe the handling and forming of the varnish before attempting to spray any art. Once adjustments are made, spraying is begun at a far corner of the painting, continuing across the surface in an uninterrupted linear pass. Constant distance is maintained (e.g., 4 to 48 inches) and spraying proceeds past the outer edge of the painting, where a turn in direction takes place for the next, slightly overlapping, parallel pass of varnish. Spraying continues in this manner until the entire surface is covered. For subsequent varnish applications, the spray orientation may be switched by 90 degrees (e.g., horizontal passes for the first coat, vertical for the second, and so on) to ensure uniform distribution (Stoner and Samet, Notes from the paintings block).

To make a sprayed varnish more matte:

stand farther away, or
increase air in spray, or
decrease size of varnish droplets at nozzle opening, or
increase viscosity of spray solution, or
add microcrystalline wax, or
use a fast drying diluent, or
chill the spray can, or
use a different resin, or
add a fumed silica (such as Cabosil®) (Leonard, Getty Museum, 1996).

To make a sprayed varnish more glossy, do the opposite of all of the above (Bernstein 1992, 111–19). If studio conditions are not ideal, allow the painting (and studio) to reach normal room temperatures. For certain paintings, it is actually beneficial to gently prewarm the painting surface, particularly oils on nonabsorbent hard surfaces, such as metal (copper or tin) or wooden panels (wood, laminate, or hardboard). Also, on a cold day, one might carefully warm the varnish solution in a bain marie (warm water bath). Precautions must be taken that there are no open flames, active heating, or hot elements in conjunction with open varnish in the studio. Always preheat the water prior to opening the varnish, remove the pot of water from the heating source and shut off the heat, and then perform the warming in an explosion-proof spray booth or vacuum hood. Carefully warm the varnish solution in a covered metal spray container, with the cover seal left slightly open to prevent vapor pressure build-up.

Another tip is to place a slightly larger cardboard behind the painting, extending beyond the outer edges of the picture. This will slow down the evaporation and improve glossing at the edges, which often dry before the wetter center of the painting has had a chance to set.

Other factors that affect spray results are the amount of air movement in the spray booth or outdoor setting (for those who spray out-of-doors) and the ambient temperature and humidity conditions. It is important to observe the weather and air conditions in your locale prior to commencing with any resin coating application. For example, on a cold or humid day, the amount of air (cfm) and pressure (psi) must be increased as varnishes would otherwise be slow to dry. Conversely, on a warm or dry day the amount of air and pressure must be decreased to allow sufficient film-forming before the solvent evaporates.

Light brushing with a dry badger-hair brush over the surface is also possible immediately after spray applications of resin, assuming that the retouching is not soluble in the diluent. This tends to remove excess varnish from the surface, and can help to create a more uniform effect.

Wiping

1. Cotton wrapped in silk or chamois can be used to rub a varnish on a painting. This is particularly useful for paintings which are highly varnish absorbent or for local varnishing on a painting with localized areas which are highly varnish absorbent.

2. A traditional method for varnishing panel paintings is to wipe on the varnish using a natural sponge. This method is more fully described in an article by Joan Reifsnyder (Reifsnyder 1996, 120–2).

Manipulation of Varnish After Application

1. It is important to remember that there are actually a number of stages of drying within an applied varnish film, which progress from the first few minutes of rapid evaporation of solvent and preliminary setting of the film, to the gradual curing and hardening that takes place in successive days, weeks, and months. The degree of solvent retention and “set” at any given time in the life of a recently applied varnish must be judged and taken into consideration when any manipulation of the surface is attempted (Bernstein 1992, 116).

2. Brushing a sprayed low molecular weight varnish with a dry, badger-hair brush immediately after application will remove excess varnish (Leonard).

3. Using a low molecular weight varnish in fine crystals to rub down an overly shiny finish, ridges, or hot spots will matte them down (Stoner).

4. Covering a brush with a velvet cloth and buffing the surface of a painting will reduce its shine (Robinson 1995, 14–15).

Individual Practices and Tricks of the Trade

The following section recounts the varnishing methods of various conservators although it is in no way all-inclusive. They can be broken down into those who use a single varnish from beginning to end and those who employ specific sequences and interlayerings. Some conservators employ mixtures of varnishes to a specific end. This section will relate not only specific varnishing techniques but the logic behind them as well.

Canvas Paintings

a) MS2A® (Gordon 1996)

In both general and local application, Gordon feels that the most preferred and commonly used varnish is MS2A®. Under some circumstances damar is used, e.g., when the painting is in good condition and has a high proportion of intensely dark areas which would benefit from optimum saturation.

Method - Before varnishing, Shell Sol® 71 is brushed over the surface of the painting to determine any noticeable differences in absorption. MS2A® varnish is used on canvases and panel supports using slight variations on the recipes of Herbert Lank. The varnish is heated in a double boiler, with Shell Sol® 71 as the diluent, to obtain complete mixing and improve penetration. Tests are done to evaluate the effects of different concentrations ranging from 7% to 25%. If a painting has areas which are very absorbent, one or more coats are locally brushed prior to an overall application, thus increasing the likelihood of achieving an even and consistent appearance. The general varnish is brushed on in a pattern of large overlapping squares. If the final effect is still uneven the MS2A® varnish can be used again to further work up areas which look dry.

Damar formulation would be used in solution including equal parts of Stoddard Solvent® and Shell TS28 (75% aromatic) and applied similarly to the MS2A®.

b) Spraying Canvas Paintings (Gordon 1996)

Canvas paintings are sprayed to correct defects in a varnish appearance when further brushing might disturb previous layering (Gordon).

c) Brush Varnishing a Final Coat Without Disturbing Underlying Retouchings

(Robinson 1995, 14–15)

“With the correct skilled technique, retouchings in the same medium can be over-varnished without being disturbed…. The best results, that is the greatest saturation with the least gloss, are obtained by the application of several very thin coats rather than fewer heavier ones. It is particularly important when a cleaned picture has uneven absorbency to keep to thin coatings. The first coat and the final one need not normally be heavier than the intermediate ones. This also does away with the ‘make or break’ final varnish because the final finish is the product of several very thin coats.

The traditional artists' colourman varnish brush is thin and soft and is useless for our purpose. The ideal brush is springy and powerful; in the case of a brush with an oval ferrule, a suitable brush would measure two inches wide by seven eighths of an inch thick with the bristles two and a quarter inches long (50 mm x 22 mm x 57 mm). The bristle must be white and of the highest quality. A springy powerful brush is needed when brush varnishing to make impossible to distribute the varnish evenly and thinly with the least possible brush strokes. If an even coat can be achieved without rebrushing over wet varnish then restorations made in the same resin will not be disturbed.

The most suitable varnishes for brush varnishing are those made with soft resins. In the past this would have meant damar or mastic, and more recently MS2A®, Ketone® N or mixtures containing these.

Method - Pour … varnish into an enamel mug; thin it with suitable thinners (use spirits of turpentine with damar or mastic and Stoddard's solvent or similar with Ketone-based varnish.). Stand the brush in the varnish to saturate it thoroughly. Find a position where the picture can be held securely with the light coming towards you across the picture. Wipe the brush out very thoroughly on the inside of the rim of the pot. It is very important to do this as completely as possible, otherwise it will be impossible to control the application of the varnish. If necessary, the brush can be wrapped in a clean lint-free cloth and squeezed to further reduce the amount of varnish in it. With the charged brush, start at the right hand far corner (for a right-handed person), keep the wrist rigid and, by partially rotating the forearm, apply the varnish, working from right to left. The brushing should be done firmly but without undue pressure; when the other side of the picture is reached, start again on the right hand side overlapping only just enough to blend the varnish into an even layer…. The coating can be adjusted by increasing the frequency of the strokes, by pressing more firmly or by altering the angle of application. All this can be done while following the systematic brushing pattern. Subsequent coatings can be applied with the picture turned through ninety degrees. If necessary, absorbent fillings can be given extra thin coatings of varnish with a small sable pencil between varnish layers to match the surrounding gloss.

Surface with strong impasto: Use a well wiped out brush more upright than usual taking care not to allow pooling of the varnish in the hollows. The secret is to wipe the brush out very well.

Normal canvas and impasto texture: Use a well wiped-out brush at a lower angle.

Smooth surfaces, for instance copper or wooden panels: Use a well wiped-out brush firmly, at a low angle, keeping rigidly to the varnishing pattern. This is the most difficult surface to treat. In general the best results are obtained by using the brush firmly, too weak a brushing technique will result in an uneven coating…. It is a commonly held belief that it is impossible to brush varnish when the restorations are in the same resin. I hope that the description above has shown that this is not so.”

d) To Resaturate an Old Varnish (Grimm 1996)

After spit cleaning to remove surface dirt, apply a brush or spray coat of Winton® Retouching Varnish diluted 1:1 in petroleum benzine.

e) A Wax Coating (Grimm 1996)

A piece of microcrystalline wax the size of a large grape is dissolved in a jar of naphtha and sprayed or brushed over a painting. It is excellent for ease of later removal. The wax can be buffed with silk so that there is no excess. It is sometimes effective on a blanched surface.

f) For a More Matte Varnish (Grimm 1996)

Add a volatile solvent like acetone or hexane, spray at a higher pressure and with a higher air mix. The varnish cools as it evaporates and hits the surface as hard balls. It give a matte appearance without adding any milkiness to the varnish.

Panel Paintings

a) Mastic on a Panel Painting (Grimm 1996)

Using a brush dampened with turpentine, brush over the surface of the painting. As the turpentine evaporates, just as the painting is losing its shine, brush it with a solution of 10% mastic in turpentine with the addition of Tinuvin® 292. Brush this vigorously. As the varnish sets, brush with a dry badger hair brush until the varnish is almost dry and a little sticky. Allow this varnish coat to dry for several days at least. A second brush application can be applied over the first if desired. Subsequent coats of mastic can be sprayed. A light spray coat of Paraloid® B-72 over the mastic can help alleviate matte or hot spots.

b) Spray Applications (Gordon 1996)

Wood panel paintings are not given spray applications of varnish. The exception would be in the case of paintings done on copper or other metal plates because the absorbency would be much less than in canvas or wood panel paintings.

See also 1.a) (Gordon) which is appropriate for panel paintings.

Paintings on Metal (Berger 1995)

Berger describes a technique for the restoration of paintings on metal. After cleaning, he first coats the losses in the painting with poly(vinyl acetate) varnish (Mowilith in a mixture of solvents). The painting is then sprayed with a thin coat of poly(vinyl acetate) isolating varnish for protection. It is next coated with a thin layer of UVS Retouching Varnish. The losses are then retouched with poly(vinyl acetate) paints. At this point, the retouching varnish can be removed by rubbing it with finger pressure, perhaps with the addition of a little pumice or dry varnish to get it started, until the retouching varnish fractures off of the surface. The dust created by the crumbling varnish is then brushed off and only the thin isolating layer of poly(vinyl acetate) remains, and the UVS Finishing Varnish remains in the low points of the shallow losses. The procedure of spraying with poly(vinyl acetate), coating with UVS Retouching Varnish, and then rubbing the UVS varnish off the high spots is repeated until all the shallow losses in the copper panel are filled with varnish and the painting appears smooth again.

Wall Paintings

a) Paraloid® B-72 in a Dilute Solution Because of Its Stability (Gordon)

When working on wall paintings a wiping technique, use a wad of cotton covered with cheesecloth held in a pan of varnish and then wiped over very large areas. This is particularly effective where the paint surface is very absorbent. Cotton can be used inside a nylon stocking to work up an area selectively prior to subsequent overall varnish application. Using slow-evaporating solvents gives one more leeway in the application—uneven buildup and overlaps are easily avoided.

Interlayered Systems

a) Combining the particular qualities of various varnishes by sequential applications is a common practice. In general, most conservators spoken to brush a highly saturating, and/or more brittle layer directly on the painting (e.g. damar, mastic, MS2A®, Arkon® P-90, Regalrez® 1094, etc.), work up the desired gloss prior to or during the inpainting process, then finish with a sprayed coating of a more durable or stable but possibly less aesthetically pleasing resin, usually Paraloid® B-72. Several other very interesting interlayered approaches were also noted.

b) Mark Tucker of the Philadelphia Museum of Art often uses a very different approach. As his first layer he brushes out Paraloid® B-72. As this is often a difficult varnish to brush, he finds that adding Aromatic® 100 to a stock solution of Paraloid® B-72 in xylene improves brushability, gloss, and saturation. The solvent is slow evaporating and the surface stays wet longer although the brushing characteristics are still somewhat peculiar. By starting with a good vigorous brush coat of Paraloid® B-72, Tucker feels that he has a good mechanical barrier between the painting and subsequent layers and that the addition of Aromatic® 100 usually improves the saturation of B-72 enough to provide a good basis for inpainting. (Note that when adequate saturation cannot be achieved with the Paraloid® B-72, damar is usually his varnish of choice.) Inpainting is then carried out in pigments ground in poly(vinyl acetate). A final brush applied coat of damar varnish with Tinuvin® 292 can then be used and manipulated to achieve proper saturation and gloss. This system allows for a final brush coat, avoiding a sprayed look and even possible reticulation on panel painting, without disturbing earlier layers or inpainting based on mutually exclusive solubilities. If the upper coat of damar (usually applied in a 20% dilution) is unsatisfactory, later coats can be manipulated in several ways. Brushing a coat of damar to complete dryness lowers its gloss. Matte spots can be corrected with local brushing or spraying or, of course, the painting may be sprayed overall. Tucker finds that a great deal of the success of this method relies on even cleaning of the painting before varnishing. No pools of varnish can be left anywhere. Where possible he now likes to leave a very thin layer of the old varnish behind which may involve picking the varnish up during cleaning and putting it down elsewhere as required for an even surface. This is particularly difficult on paintings with texture.

c) A Varnishing Technique Used By Mario Modestini

A painting is brushed with Talens Rembrandt® Retouching Varnish (Laropal® K80 in white spirit). This can be diluted with purified turpentine or mineral spirits, or made more viscous by adding Talens Rembrandt® Picture Varnish, the same resin in a higher concentration depending on the absorbency of the surface. Usually just enough varnish is applied so that the colors are sufficiently saturated to match when retouching. After the retouching is completed, any matte areas are brought up to surface gloss by locally varnishing with either the medium, PVA AYAB in alcohol or the substitute, PVA Mowilith® 20, or with retouching varnish.

The surface is then sprayed with a solution of PVA AYAB or PVA Mowilith® 20 in alcohol and acetone. (Note: These resins do not dissolve easily in ethanol. Usually they are dissolved first in acetone and then diluted with ethanol. If the solution becomes slightly cloudy when diluted in ethanol, add a little acetone. Common hardware store “denatured alcohol” dissolves the resin easily.) The solution is approximately 5%. It can be checked by painting out on the fingernail, where it should leave only a slight gloss when dry.

This is sprayed on the surface, a light spray, a few times. It dries almost instantly. If it leaves resin dust on the surface, the gun is too far from the surface or the air pressure setting is too high. On the other hand, the varnish should not wet the surface. As with any other spray, the distance and the amount vary with the size of the painting.

After 24 hours, more or less, depending on the weather, the surface can be given a second brush coat or varnish, the viscosity chosen according the characteristics of the painting and the surface desired. If necessary, these steps can be repeated to build up a third brush varnish coat.

One advantage of the technique is that the isolating spray provides a sort of floor, evening out different absorbencies of the surface, allowing a leached or damaged surface to hold a brush varnish, sealing up uneven cleaning which had to be suspended for some reason or left partially cleaned. It basically gives an evenly sealed, evenly absorbing source and allows the conservator to apply a second or third varnish coat without picking up the previous varnish. This is not otherwise possible with any resin except mastic. Another advantage is that the restorations “stay put”; that is, they will not later either drop, become matte, or become glossier than the rest of the varnish. In a survey of paintings in 18 Kress regional galleries completed a few years ago, paintings varnished using this technique had held up exceptionally well. After nearly 30 years, the surfaces had retained a pleasant sheen and the varnish had not noticeably discolored. (The varnish used was always Talens Rembrandt®. The resin has varied over the years but seems always to have been a polycyclohexanone, e.g., AW2®, Ketone® N, or, recently, Laropal® K80.) By comparison, paintings varnished a few years earlier by the same conservators using damar varnish showed significantly more yellowing.

The fact that the varnish showed so little discoloration over the years is partly due to the fact that the new varnish applications were minimal, another advantage of this method. Except in unusual circumstances, the total amount of varnish used was two brush coats of retouching varnish with a sprayed interlayer of 5% PVA AYAB (Dwyer 1992, 121–2).

d) A technique employing alternating layers of Paraloid® B-72 and B-67 was used until 15 years ago for achieving more even surface gloss where appropriate, and still may be useful. A thin, isolating brush coat of B-72 was applied to a cleaned painting, and inpainting was carried out, often with PVA as a medium. If there were serious problems of uneven gloss, very thin, alternating spray coats of B-72 and B-67 would then be applied, with a week's waiting period between the application of each one, if possible. This produced a refined surface appearance, although it was often not appropriate for paintings requiring very good saturation of darks. The appearance of paintings varnished with this technique is still acceptable after 15 years, although questions of long-term adhesion of the two different layers to each other have remained open (Fisher 1995, reporting on past National Gallery practices).

Trouble Shooting

Reticulation

a) Usually this is solvent related. It is best to avoid the problem if possible by using successive thin coats applied with lower air pressure rather than by a single thicker coat applied with higher air pressures. Temperature may also be important. Turn the air flow in the booth to low or off. Use a Chiron® gun with the air volume way down. The combination of warm air and low air pressure should help avoid reticulation (Grimm).

If reticulation does occur, immediately get the painting off the easel, place it flat and brush with a dry, badger hair brush.

b) Reticulation is best avoided by simplifying varnish choices (Gordon).

c) To avoid reticulation, add a dollop of acetone to your varnish mixture (Stoner recounting a technique of Bernie Rabin).

d) Reticulation is best avoided by ending with a brushed rather than a sprayed coating. Allow varnish coats to dry well between applications and use thin coats (Tucker).

Ridges of Excess Varnish

a) Make a HARD swab and dip it in a solvent like acetone. Let it dry until just damp and rub it on the ridge. There can be no loose fibers in the swab. One can dull hot spots in the same way (Stoner recounting a technique of John Brealey).

b) Buff a local ridge or hot spot with a swab made of 00000 steel wool (Stoner).

To Correct an Overly Glossy Varnish

a) Insert a wad of cotton into a length of silk, pulling it tight so that there are no creases. This silk can then be used to polish the varnish (Grimm).

b) Buff with varnish crystals (Stoner).

c) Apply a very thick layer of Paraloid® B-67, buff the varnish with 00000 steel wool, and revarnish with a thinner coat of Paraloid® B-67 (Stoner recounting a technique of Bernie Rabin).

d) A cheap quality tissue paper with extremely short fibers (not Japanese tissue!) can be used to buff down a too glossy varnish (Stoner recounting a technique of Andrea Rothe).

Shiny Spots, “Hot Spots”

a) These can be buffed down or simply touched up with a little wax to reduce the shine. A gentle and well-placed fingertip can often be just enough to impart a print which will break up the glossy patch (Gordon).

b) Buff with varnish crystals (Stoner).

c) Apply a very thick layer of Paraloid® B-67, buff the varnish with 00000 steel wool, and revarnish with a thinner coat of Paraloid® B-67 (Stoner recounting a technique of Bernie Rabin).

d) For smaller hot spots, Paraloid® B-72 with acetone in the dilution can be airbrushed or sprayed. One must work very close to the surface. For larger hot spots one can try various spraying tricks or stippling with a brush (Tucker).

Local Varnishing

Local varnishing is done at least a day after the inpainting is executed. In some rare instances varnish may be used in mixture with inpainting pigment to avoid creating mismatched levels between an area of retouching and the surrounding original paint. For local varnishing a small fan brush or inpainting brush is used, depending on how large the area to be covered is (Gordon).

To Even Out a Varnish

a) Spray a mixture of ethanol and mastic in a very dry and pebbled manner. The painting will appear covered with snow. Then apply a saturating spray coat of varnish which will reform the layer below and leave an even coat overall (Grimm).

b) Varnish can be applied using squares cut from old sheets or a chamois (Grimm).

Special Circumstances

Acrylic Emulsion Paintings

(see following section)

The Use of Paraloid® B-72 as a UV Barrier Top Coat (James Bourdeau)

In ultraviolet-containing environments, in museums, historic houses, or private collections with poor lighting control, the yellowing and degradation of damar picture varnishes can be slowed by top coating the painting with an ultraviolet barrier acrylic varnish. The following is a summary of the technique for applying an ultraviolet barrier acrylic top coat over a natural resin picture varnish. Although there are numerous types of additives which are used to block the transmission of ultraviolet, only two have direct application in acrylic barrier top-coats for paintings, substituted 2-hydroxybenzophenones and substituted 2-hydroxybenzotriazoles.

To use this method, damar in xylene is applied to a painting according to one's own preference: by brush, spray, or a combination of the two. Establishing the damar varnish gloss is very important since the B-72 top coat is intended to be spray-applied as a thin film in order to preserve the gloss characteristics of the damar. Paraloid® B-72 for the barrier top coat is prepared in any desired concentration suitable for spraying, here at 10–15% weight-to-weight in xylene. A maximum concentration of 3% Tinuvin® 327 in powder form is added to the B-72 solution. The ultraviolet absorber is always measured as a percentage of the total weight of the resin or polymer, not of the weight or volume of the solvent.

For example:

UV Barrier Acrylic Top Coat Solution

10% Paraloid® B-72 solution

Paraloid® B-72	40.0 g
solvent (xylenes)	360.0 g	}400.0 g total solution
add-Tinuvin® 327	1.2 g	(3% UV absorber, weight-to-weight to polymer)

It is important that the concentration of the additive not exceed 3% of the weight of the polymer since it can separate from the dry film in a whitish efflorescence at higher concentrations. The additive may also be added to the solvent before adding the polymer as long as the concentration of the additive is measured in relation to the mass of the polymer. The ultraviolet barrier acrylic varnish can then be spray applied to the “dry” natural resin film. A good practical guide during the ultraviolet top coat varnish application is to periodically examine the painting with ultraviolet fluorescence lamps as successive, thin layers are applied. Successive top-coats are applied until the ultraviolet fluorescence of the painting's surface is reduced to a minimum, i.e., the surface looks dark.

The desirable top coat thickness is in the range of 3 to 4:m (micrometers) or about 15 spray passes depending upon spray pressure and distance from the surface. (Films of 3–4 micrometers were measured as 12–15 spray passes at 40 psi and 1.0 micrometer distance from the surface.) A difference in the top-coat's thickness of even one micron can affect the yellowing of the damar below after years of ultraviolet exposure. Because we are limited by the efficiency of the absorber (its ultraviolet cutoff and extinction coefficient) and its concentration (no higher than 3%), the only degree of protection we can control, in environments where ultraviolet is present, is the barrier film's thickness.

Because of the possibility of solvent residues remaining in the damar varnish substrate, it is very important to use a solvent which would be compatible with the B-72 top coat. Many conservators prefer to use mineral spirits at about a one-to-one mixture with an aromatic solvent. If an acrylic top coat is applied to a damar substrate which contains residual mineral spirits, the solvent can migrate into the top coat and, because the acrylic precipitates in this solvent, fogging will occur at the interface of the two coats. If mineral spirits or turpentine are used in the natural resin substrate, this varnish must be allowed to off-gas until none of the solvent remains.

Ultraviolet Examination of UV Absorbing Top Coat

The amount of ultraviolet radiation blocked by an ultraviolet barrier top coat is limited by the additive's concentration, its extinction coefficient, and the thickness of the film, as mentioned above. This threshold limit to a film's ultraviolet absorption can be surpassed by increasing the intensity of the ultraviolet source. The surface of a painting with an ultraviolet barrier varnish can still be examined with ultraviolet either by holding the ultraviolet light source closer to the surface, which effectively increases the ultraviolet intensity, by using a higher wattage radiation source, or better still, by ultraviolet fluorescence photography which uses a longer exposure time to record the weak fluorescence at the surface of the painting. Good photographic results have been obtained by decreasing the distance of the ultraviolet source to the painting's surface by half and, with a constant camera/stop, by increasing the photographic exposure time by a factor of four (Bourdeau 1995a).

References

Berger, G.A. 1995. Inpainting media and varnishes which do not discolor, part 1: Preparation for inpainting. The Picture restorer 8:5–8.

Bernstein, J. 1992. A Review of varnish application fundamentals. In 1992 AIC Paintings Specialty Group Postprints: Papers presented at the Twentieth Annual Meeting of the American Institute for Conservation of Historic and Artistic Works, Buffalo, New York, Saturday, June 5, 1992. Washington, D.C.: American Institute for Conservation of Historic and Artistic Works: 111–19.

Bockrath/Bull. 1995. Personal communication.

Dwyer, D. 1992. A Varnishing technique used by Mario Modestini. In 1992 AIC Paintings Specialty Group Postprints: Papers presented at the Twentieth Annual Meeting of the American Institute for Conservation of Historic and Artistic Works, Buffalo, New York, Saturday, June 5, 1992. Washington, D.C.: American Institute for Conservation of Historic and Artistic Works: 121–2.

Gordon, E. [n.d.] Personal communication.

Grimm, C. [n.d.] Personal communication.

Keck, C. and Buckley, B.A. [n.d.] Personal communication.

Leonard, M. 1996. Personal communication.

Lomax, S.Q. 1995. How to calculate concentrations of solutions. AIC news January 1995).

Reifsnyder, J.M. 1996. A Note on a traditional technique of varnish application for paintings on panel. Studies in conservation 41:120–2.

Robinson, P. 1995. Brush varnishing at the Studio of J.H. Cooke & Sons. The Picture Restorer 8:14–15 (Autumn, 1995).

Stoner, J.H. [n.d.] Personal communication.

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