Interior Paints for Exhibit Cases

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Contributors: Members of the MWG's Materials Fact Sheet Working group, Jenifer Bosworth, Anne Ennes, Lisa Goldberg, Jamie Gleason, Jennifer Herrmann, Nancy Lev-Alexander, Patricia Silence, Catherine H. Stephens, and Theresa Voellinger

How are paints used in exhibits?

Paints are used for aesthetic reasons to enhance the exhibit design. Paints can be used on exhibit walls, case exteriors, or case interiors; however, care must be taken when using paint within an enclosed case. Paints do not create a vapor barrier to stop emissions from other construction materials, such as wood.

Why are paints a preservation concern?

The selection of paints used in exhibit cases is an important preservation decision because the high level of emissions produced by most commercial paints may interact with the collections displayed. Systems based on fully cured water-based acrylic resins with zero or low volatile organic compounds (VOC)* emission rates and epoxy paint systems are recommended, although many of these products do emit volatiles. Preservation recommendations of four weeks to minimize residual volatile emission or allow for full cure may be longer than manufacturer product information. Drying/curing time will depend on the environmental conditions found within the space.

Specific concerns

• Many paints emit volatile organic compounds (VOCs) that are harmful to objects including organic acids, aromatic hydrocarbons (such as benzene and toluene) and chlorinated hydrocarbons (such as methylene chloride).
• Combining evaluation of the ingredient list with a measurement of the coating's VOC level is a good way to determine whether the paint is safe to use in an exhibit case. A low or zero VOC content is desirable, and is measured in "grams of VOCs per unit volume.”

The U.S. Environmental Protection Agency regulates levels of VOC emissions in paints (Federal Register, section 59.402). VOC contents are tested by the EPA's Reference Method 24. Paint labels display the "grams of VOCs per liter." The specific definition is "grams per liter excluding water or exempt compounds, thinned to the maximum thinning recommended by the manufacturer" (Proposed Rules of the Federal Register, vol. 61, No. 171, September 1996). A rule of thumb is the stronger the smell of paint fumes, or the higher the gloss level, the higher the VOC content.

• For information on environmental considerations, see Sustainable Use of Coatings in Museums and Archives – Some Critical Observations in Additional Resources
• Paints should not be used as a barrier to block VOCs from other building materials in a display case.

• Alkyd paints are known to offgas harmful VOCs, even though they are considered to be more durable by some users.
• Acrylic latex paints can remain tacky for long periods of time because they take a long term to dry (and especially during periods of high humidity); always isolate collection objects from painted surfaces.

• A wide range of VOCs are now detectible by various analytical methods, complicating the picture of what paints are acceptable for use inside an exhibit case.

What types of paints can be recommended?

A paint system generally consists of a solution composed of pigments, a binding medium, and a solvent (or vehicle), although small amounts of many other ingredients are added to affect viscosity, flow, gloss, drying time, final hardness, and other features. Discussions around paint formulations revolve around VOCs, how long they are released and durability of the paint layer. Any solvent-based paint used in close proximity to exhibit objects can be considered problematic due to the harmful effects of VOCs given off by these coatings. Careful choice of paint systems, appropriate materials testing, and generous drying time will all contribute to safe display environments. For example, avoiding paints with low pH can help reduce offgassing of acidic compounds. Direct contact with any painted surfaces is not recommended due to the possible transfer of components.

Paints are not effective in blocking the migration of volatile organic compounds; all paint formulations should be evaluated for barrier effectiveness. Latex paint does not create an effective vapor barrier coating for wood products because the coating is permeable to wood offgassing and formaldehyde emissions. If you are choosing to use a paint for aesthetic reasons, consider covering the wood with an impermeable layer such as aluminum composite material (ACM), or use of a barrier paint film such as a neutral cure, marine-grade epoxy paint. Metal laminate film can be used to seal wood-based components that are not visible as the laminate cannot be painted.

General considerations to follow in choosing a paint

• Additives: Evaluate additives which may be present in paint such as mold retardants, plasticizers, flow modifiers, and solvent components.
  • Paint containing calcium carbonate offers some benefits when acids are a particular concern.
• Acrylic latex paints (emulsions) are dispersions of pigments in a liquid continuous phase of water and an acrylic resin, prepared by emulsion polymerization. Latex paints vary in formulation and may contain ammonia, sulfur or other undesirable chemicals.

• Resin type: Oil and alkyd-based binders in paints are not appropriate for use inside an exhibit case.
• Remember that paint formulations change frequently; check in Additional Resources or manufacturer technical specifications (TDS) and Safety Data Sheet (SDS) for specific information before you use any product.

Summary of Recommended Paint Types

Note that information relating to specific products (especially description and working properties) can be found on Cameo.

Category	Description	Use Considerations
Acrylic Latex	Aqueous emulsions of synthetic resins that dry by water evaporation.	Drying time recommended: 4 weeks to minimize collection risk
2-part Epoxy	A film formed by catalyzed polymerization and solvent evaporation.	Stability is dependent on careful mixing and application. Curing time recommended: follow product use directions and then allow to offgas if any smell is noticed
Powder coating (polyester-based)	A thermosetting resin applied by electrostatic spray deposition to evenly coat metallic pieces without the use of solvents	Use requires lead time in planning because the paint must be applied by the manufacturer. There is no need for further dry/cure time once powder coated components are received.

Preservation strategies

• Consider the vulnerability of the collection items (in contact or enclosed in a case).
• Consider the duration of the exhibit; paints are often used for short-term exhibits.
• Remove objects from the painting area during application.
• Never place objects inside the case until the paint is thoroughly cured. A four week interval is recommended to allow all case materials to offgas.
• For more information about specific paints and their role in exhibit preservation, see Additional Resources

*Carbon-containing molecules that have a high vapor pressure at room temperature are referred to as volatile organic compounds (VOCs). Low levels of VOCs in paints, adhesives, caulks, carpets, etc. normally makes one think that the material does not offgas. This is not technically correct in the United States. The Environmental Protection Agency (EPA) has designated a list of volatile organic compounds that can lead to eye, nose, and throat irritation, headaches, damage to the central nervous system, and cancer, but many compounds have received exemptions. To qualify as low-VOC under currently accepted standards, interior paints (including primers or other coatings) have to have fewer than 250 grams per liter, while non-VOC products have less than 5 grams per liter (California Air Resources Board 2022). Most paints that qualify as low-VOC are latex or water-based.

Additional Resources

• Low VOC paints - CAMEO
• Materials Testing Results on the AIC Wiki
• The use of commercial paints in museums - Canada.ca
• Products used in Preventive Conservation - Technical Bulletin 32. by Jean Tétreault
• Green Seal Paints, Sealants and Adhesives
• Tétreault, J. 2011. Sustainable Use of Coatings in Museums and Archives – Some Critical Observations. e-Preservation Science 8: 39-48. This resource is critical for a complete understanding of preservation strategies surrounding effective and safe paint use.