Varnishes
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(Excerpts from ART HARDWARE: The Definitive Guide to Artists’ Materials, by Steven Saitzyk © 1987) Unprotected polymer paint films are not very durable because they are relatively soft compared to oil paint films and can be marred easily as the result of abrasion. They are also more porous and the pigments and paint film can be affected by air pollution. A varnish can, however, provide the necessary protection. A varnish can also be used to control the surface appearance in terms of matte and gloss. Acrylic polymer solution varnishes, which are dissolved in mineral spirits, are the best protection for acrylic paint films. They are more durable, forming a harder protective film than acrylic emulsion varnishes, which are waterborne. Acrylic polymer solution varnishes also provide better protection against air pollutants because they seal the painting's surface more thoroughly. An acrylic polymer emulsion varnish is the same as an acrylic polymer emulsion used as medium, except that the varnish contains a hardener. Unless specified by the manufacturer, an acrylic varnish should not be used as a painting medium. Most manufacturers discourage the thinning of acrylic varnishes, and if diluted with more than 50 percent water, the clarity of the dry varnish film can be seriously affected. The application of a varnish should be smooth and consistent, and areas that might require more varnish should not be reworked. Reapplying a varnish to a partially dry area can result in streaking, evident brush strokes, and a milky appearance. It is best to wait the hour or two until the varnish dries, and then apply a second coat. Matte varnishes are particularly susceptible to this problem. The painting surface shine can be regulated not only by choosing a matte or gloss varnish, but also by using a mixture of the two. A matte varnish, despite the hardeners added to it, is not as durable as a gloss varnish. And the matte surface can be rubbed or polished to a shine.
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