The subject of selecting varnish is important because the decision can result in a painting being protected and long-lasting verses a painting that is destined to have trouble. Hard to believe? Well, here are some issues regarding varnishing.
Art conservators know all too well the consequence of using the two historically dominant natural resin varnishes. Damar (no comment at this time on spelling this resin with one or two "m"s) and mastic have been the two resins used extensively for several hundred years. I am not including egg whites since it is not a resin. Varnish has even made its way as an ingredient in painting mediums. The classic thirds-medium (1/3 turpentine, 1/3 linseed oil and 1/3 damar varnish) results in paint that yellows and is difficult to clean over time.
Old paintings that have not been treated display a dark brown-yellow film that obscures the true colors beneath. The brown-yellow film acts like a filter that, for example, changes blue to appear green and sullies all other colors to a great degree. When viewing yellowed paintings due to varnish discoloration, our brain compensates for the color shifts and the picture merely appears to display a warm “patina” of age.
So, if mastic and damar are not suitable for long-term protection that avoids discoloration, what are the alternatives? A great deal of painstaking research was conducted at the National Gallery of Art several decades ago and elsewhere on substitutes for natural resin varnishes.
The “trick” is to find a resin that, when dissolved, will mimic the molecular weight of a natural resin. When varnish is applied to a painting it appears wet, once dry, the “wetness” remains and is visually translated into the appearance of saturation of the paint. The color appears to have greater intensity, especially if extensive sinking in of the paint has caused the pigment to look dry and blanched.
We appear to gravitate toward shiny things. Notice how in many scenes in films shot outdoors have the streets and sidewalks appear wet despite no evidence of rain. Film makers like saturation. I imagine on a long day of shooting a film, the number of times a water truck has to respray a street to maintain a wet look. The same phenomenon is experienced with paintings. Saturation is appealing.
When we think of how a painting appears under a microscopic in a side view of the work of the topography, it looks like a mountain range, filled with hills and valleys created by the action of a brush and surface texture of the substrate upon which the paint is applied. Natural resin varnishes fill in the valleys with resin and do not leave voids in the deepest areas that cause the painting to look hazy. The voids don’t reflect light the same way the resin coated areas do and the voids produces a somewhat cloudy appearance.
To the rescue came synthetic resin varnishes. The size of the synthetic resin molecules are as small as the natural resins and fill the valleys in a similar manner. Several have been adopted over the last 2 decades that are good substitutes for natural resin varnishes. They saturate well and they are removable with mild solvents so that when they eventually discolor over time (a far longer lifespan than natural resin materials) they can be removed from a painting.
What NOT to choose: In my opinion, do not be lured by the “amazing” level of gloss and saturation produce by epoxy resin. It is not a suitable varnish for any work of art fine art. (Note: I am making a distinction between fine art and craft/hobby projects.) Epoxy resins cannot be removed. Epoxy was developed to be permanent and impervious to solvents, humidity and any kind sort of chemical removal. Artwork subjected to epoxy coatings are permanently entombed. Like many coating materials, epoxy resins will eventually discolor or become hazy. It is an inherent property of this and a host of other surface coatings. In most applications, like bar-tops or outdoor settings where a rock-hard coating is required, the substrate is likely be discarded and replaced or be subjected to a belt sander and then re coated. I don’t believe you wish to use the same strategy and cleaning method for treating a painting.
While not having done a survey, I would safely say that no conservator would advocate the use of epoxy resin as a surface coating for a work of fine art. The only excepting might be an outdoor or indoor mural in a heavily traffic space where the artwork is considered somewhat temporary and will be subjected to the elements or high wear. (The elements can include rain, lots of finger touches, handbag and briefcase hardware scraping the surface of a mural. (Think of a decorative mural in a pedestrian walkway within a subway station.)
Epoxy resins are wonderful for craft projects where hard, shiny protection is desired and no expectation exists that the project will be kept for generations. These resins can be thinly applied or used as casting materials to create amazing depth and dimension. They are fun to use with crafts. Remember to use proper ventilation. Epoxy hardening is a chemical reaction that generates fumes.
In the end, one might choose to not varnish a work of art at all. That is a suitable alternative. However, if selecting a varnish for a work of fine art, look toward the synthetic resin varnishes that were developed for artworks.
The Syntax of Color