In August 2001, the Smithsonian Center for Materials Research and Education (SCMRE) in conjunction with the traveling exhibit “Santos: Substance & Soul,” presented a 4 day seminar cosponsored by the Smithsonian Center for Latino Initiatives, College of Santa Fe, the National Hispanic Cultural Center, and the Museum of Spanish Colonial Art. As part of a team of presenters including leading artists, conservators, art historians and paint manufacturer Robert Gamblin, I offered a lecture centered on the issues associated with acrylic paint performance on wood. Preparing samples and getting ready for the presentation, the Research and Development team at Golden had an opportunity to explore the issues that many artists deal with in working with acrylics on wood surfaces. I personally had the opportunity to learn about the incredible range of materials now used by so many of the Santeros working in this traditional devotional form of art. I thank the many presenters for their vast knowledge, dedication and artistry and especially SCMRE and project leader, Jai-sun Tsang, Senior Paintings Conservator, SCMRE for allowing me to share in this event.
Acrylic Adhesion on Wood
Acrylics have been a recommended choice of paint for a wide range of artists and artisans working with wood. We have had numerous opportunities to engage with artists working in wood sculpture, carving, furniture and wooden architectural and functional objects, as well as wooden panel painting. In each case the acrylic emulsion paint has provided exceptional working properties, including excellent adhesion, excellent pigment clarity and lightfastness. We have, during our recent research, made a closer examination of the properties and potential of acrylic paints and mediums on wood surfaces.
The most obvious property to discuss is the considerable difference between the emulsion acrylics, which contain water, and the solution acrylics, which are mixed with solvents such as mineral spirits or alcohol, not containing any water.
The acrylic emulsion polymer contains at least 40 percent water before it is formulated into paint. Water may be added to wet out additives and pigments. In every case where water is present in the primer or paint, the water contributes to raising and swelling the wood grain. Even waterbased products recommended specifically as sanding sealers for wood show significant raising of the wood grain. Once the acrylic emulsion paint or medium has swollen the wood, subsequent applications of the waterborne acrylic do not have the same dramatic effect on raising the grain. The dried acrylic tends to hold in place and bonds extremely well to the open grain.
An artist creating work with very fine detailed carving such as hair, feathers, etc. will lose a significant level of detail if a waterbased primer sealer is used. If this level of detail is not required, then the acrylic can be painted directly onto the wood surface. Acrylic emulsion paints (or what we commonly refer to as just acrylics,) will swell the wood. If this does not detract from the work then it is a very appropriate way to begin. In fact it is possible that the swelling insures great contact and closure of the fibers around the acrylic matrix. When sanding is possible after the first coat, additional coats will create less noticeable texture in the grain.
To avoid raising the wood grain completely, solution acrylics such as the MSA varnish or colors (thinned with mineral spirits) or pigmented shellacs will work quite well. These will offer a sufficient base to then apply the emulsion paints, reducing the effects of raising the grain. Our studies have shown that the nonpigmented shellacs do not offer the same level of adhesion to subsequent applications of acrylic waterborne paints. If it is necessary to use these nonpigmented shellacs in order to maintain some of the wood color, then adhesion can be dramatically improved by sanding the shellac after it has dried. Artists should avoid using multiple heavy coats of shellac.
Another issue facing artists working with wood is the significant staining that can discolor painted surfaces. Not all woods stain to the same level. These effects were quite evident in some woods including cedars and red oak, and with particularly highly staining woods such as orange osage and mahogany. Neither emulsion acrylics nor solution acrylics systems were capable of resisting the discoloration that was caused by oils and water miscible components in the wood being transported through to the surface of the coating. The color change was most noticeable in transparent acrylic colors. Even more disconcerting is some of the blotchy color staining that occurred on opaque colors.
With these woods the most effective stain blocking was produced by pigmented shellac systems. Commercially these are available in brands such as BIN® and Kilz®. These primers raised the grain very little and were very easy to sand. They are not perfect solutions to the staining of the color but they dramatically improved the situation.
It is understandable that acrylic gesso made with the same waterborne acrylic emulsion will contribute to swelling the wood fibers and will not block the staining. These materials remain porous, allowing water to pass through the film in both directions. In fact acrylic gesso tends to be quite porous and absorbent, thereby increasing the likelihood of surface staining. Woods such as yellow pine can be used successfully with acrylic emulsion paints without developing subsequent discolored staining.
From our work and the recommendations of others we have provided a “best practices” for painting wood: First prepare the wood by sanding or scraping to remove loose fibrous materials and to open the grain of the wood for penetration of the primer. Clean off all dust with a tack cloth that doesn’t leave a residue on the wood. Seal the wood with either a pigmented shellac, MSA varnish or solvent pigmented alkyd primer. Let dry and lightly sand. Apply two coats of acrylic gesso for best hiding and surface protection, sanding, if desired, between coats.
At this point any well-made acrylic paint will adhere to the surface. In fact it is likely that an acrylic paint will pull the wood fibers apart before failing.
Finally, some recommendations for working with the acrylic paint: If this surface appears too absorbent for your needs, add additional acrylic polymer to the gesso mixture, or skim coat with a soft matte or semi-gloss gel. This will allow the colors to glide across the surface avoiding brush drag.
More fluid colors will tend to penetrate the wood or absorbent paint layer, but at this point it is more about aesthetics than properties of the material. A good rule of thumb is that the thinner the paint the greater the interpenetration between coats. Thinner materials will flow into crevices and bridge across surfaces, as do thicker paints.
Acrylics are generally a porous film and can pick up dirt easily when handled. A varnish is therefore an appropriate final coat, especially considering that the work may be in a humid environment subject to mold and mildew. If the artwork is also a functional piece to be handled, we recommend a solvent borne acrylic varnish. This will help the work resist dirt pickup and be more easily cleaned. If the piece is more utilitarian, then painted surfaces may have to be coated with a polyurethane. This would be a strong recommendation for a chair or any object with constant use. Objects that may be used as counters will absolutely require a urethane finish to avoid the blanching that can occur with waterborne acrylic. Some of the waterbased urethanes now resist yellowing. The solvent borne aliphatic urethanes continue to be the most protective, but extreme caution should be exercised because of the range of potentially toxic chemicals used in these urethanes. It is critical that the artist knows that this coating is not removable without damaging the paint. Use of waxes (Renaissance Wax®), or possibly other synthetic varnishes, on the acrylic may also be appropriate and has been recommended by several artists and wood refinishers.
The Santos presentation was given by Mark Golden.