Guidelines for Selecting and Using Coatings
Archived Content
Information identified as archived on the Web is for reference, research or recordkeeping purposes. It has not been altered or updated after the date of archiving. Web pages that are archived on the Web are not subject to the Government of Canada Web Standards. As per the Communications Policy of the Government of Canada, you can request alternate formats on the "Contact Us" page.
CCI Newsletter, No. 28, December 2001
Guidelines for Selecting and Using Coatings
by Jean Tétreault, Conservation Scientist, Preventive Conservation Services
The choice of suitable paints, varnishes, and stains is an important part of the planning process for all exhibition and storage room projects. There are many coatings on the market for architectural and industrial use, but not all of them are appropriate for conservation purposes. In addition to the adhesion and weathering qualities of coatings, museum and archival institutions must also be concerned about the emission of any volatile compounds that could be harmful to works of art and archival documents. For example, emissions from oil-based and fresh alkyd-based paints are known to corrode some metals (e.g. lead, copper, and zinc) and to promote the deterioration of photographs. Paper or cotton can also be susceptible to damage.
CCI often receives questions about the use of coatings for presentation and preservation purposes. In response to this need for information, a Technical Bulletin entitled Coatings for Display and Storage in Museums was published in July 1999 to provide guidelines for selecting and using coatings. The Bulletin discusses various coatings for the wood, metal, and concrete surfaces of floors, walls, and display or transportation cases. Table 1 presents a simplified version of the guidelines.
Powder coatings form a film on the surfaces to which they are applied in a process that does not require a solvent, but liquid coatings need solvents to aid their application and film formation. These solvents always release vapours as the coatings dry. Although the rate of emission of vapours decreases with time, this does not happen as rapidly as is generally thought. Because the presence of large quantities of these volatile products can be damaging for works of art (especially objects made of metal or with metal components), it is important to allow sufficient time for coatings to dry and the harmful emissions to dissipate before placing artifacts in the vicinity.
The dissipation of vapours is particularly difficult in enclosed spaces such as display cases or storage cabinets, where volatile emissions can build up to very high concentrations. The more airtight the enclosure, the longer it will take for solvent vapours to reach safe levels.
Placing metal artifacts inside a display case or storage cabinet that was painted just a few days earlier would be a very risky scenario. To avoid potential problems, a drying time of 4 weeks is required — even when recommended coatings have been used. This 4-week period corresponds to the average time it takes for a coating to release most of its volatile products. If time constraints make it impossible to wait 4 weeks, the enclosure should either be constructed from a material that does not release volatile vapours (e.g. plastic panels) or sealed with a laminated aluminum film such as Marvelseal instead of a wet coating. The drying time for a painted room is much shorter (only about 4 days) than that for a display case because the volatile products dissipate much more quickly in the room's larger dimensions.
CCI does not recommend any coatings in particular (as formulations may change) but instead provides general guidelines based on the nature and use of the coatings. By avoiding oil- or alkyd-based paints, following recommended drying times, and keeping artifacts that contain lead away from freshly painted areas, collections should be exposed to very few risks.
More information on coatings or related topics is available in the following documents. Enjoy your painting project!
Tétreault, J. Coatings for Display and Storage in Museums. Technical Bulletin, No. 21. Ottawa: Canadian Conservation Institute, 1999.
Tétreault, J., and E. Stamatopoulou. "Determination of Concentrations of Acetic Acid Emitted from Wood Coatings in Enclosures." Studies in Conservation 42 (1997), pp. 141-156.
Tétreault, J. Oak Display Cases: Conservation Problems and Solutions. Ottawa: Canadian Conservation Institute, 1999.
Tétreault, J. "Display Materials: The Good, The Bad, and The Ugly." pp. 79-87 in Preprints of Exhibition and Conservation (edited by J. Sage). Edinburgh: Scottish Society for Conservation and Restoration, 1994.
Tétreault, J., J. Sirois, and E. Stamatopoulou. "Study of Lead Corrosion in Acetic Acid Environment." Studies in Conservation 43 (1998), pp. 17-32.
Table 1
Simplified Coating Guidelines
| Wood products | Metals | Concrete* | |
| For enclosures such as display cases and storage cabinets: | Avoid acidic woods such as oak and cedar. All paints except oxidative paints are acceptable. Varnishes need several layers. A drying time of 4 weeks is recommended. | Powder coatings require a drying period of only 1 day. With two-part epoxy or (properly) baked alkyd paints the drying period must be 4 weeks. | Not commonly used for this type of enclosure. |
| For open structures such as storage shelves (no doors), walls, and ceilings: | All paints except oxidative paints are acceptable for all surfaces. Allow a drying period of 4 days. Check with the distributor or the technical data sheets to ensure that the paint is appropriate for the surface to be painted. If possible, the relative humidity should be kept lower than 65% during the drying period. | ||
| For floors (which are a special case of open structures): |
All paints except oxidative paints are acceptable for all surfaces. Be sure to select paints that are recommended for floors. Allow a drying period of 4 days (or more if specified by the manufacturer). | ||
| For contact between objects and paint film: | Wait 4 weeks before allowing direct contact between objects and painted surfaces. Four days of drying is sufficient if interleaves such as plastic sheets (e.g. polyethylene and Mylar, but not polyurethane foam or PVC) or alkaline papers are used. For metal surfaces treated with powder coatings or baked alkyd paints, direct contact is safe after 1 day of drying. | ||
| For display or storage of lead objects in a newly painted enclosure or room: | Even when using appropriate coatings and drying
periods, some lead objects or rich lead alloy metal objects
can be altered by carboxyl acid vapours (especially acetic acid)
released by coatings or wood products. It is therefore recommended that these materials be avoided
as much as possible when displaying or storing lead. |
||
*New concrete surfaces will need to be etched with a muriatic solution to improve the paint adherence. For old concrete surfaces a TSP soap (trisodium phosphate solution is a common soap for cleaning concrete surfaces) should be sufficient.
Oxidative paints include oil-based urethane, alkyds, and epoxy
ester (i.e. epoxy in one paint can).
