varnish, liquid coating material containing a resin that dries to a hard transparent film. Most varnishes are a blend of resin, drying oil, drier, and volatile solvent. When varnish dries, its solvent portion evaporates, and the remaining constituents oxidize or polymerize to form a durable transparent film. Varnishes provide protective coatings for wooden surfaces, paintings, and various decorative objects. Varnish protects and enhances the appearance of wooden floors, interior wood paneling and trim, and furniture.

The early varnishes were solutions of natural resins that are the secretions of plants. Among these natural resins are dammar, copal, and rosin (qq.v.). The natural varnishes are produced by heating the resins, adding natural oils such as linseed oil, cooking the mixture to the desired viscosity, and then diluting it with turpentine. The resultant coating took three to four days to harden, had a yellow tint, and eventually developed cracks as it aged.

Natural varnishes have largely been replaced by varnishes containing synthetic resins, chief among which are the alkyd, polyurethane, phenolic, vinyl, and epoxy resins. The first synthetic resins used in varnishes, developed by the chemist Leo Baekeland, were phenolic resins similar to Bakelite. Improved through the 1930s and ’40s, phenolics were displaced in many uses by alkyds, which eventually became the single most important resin class in the coatings industry, though phenolics continue to be used in marine and floor varnishes. Alkyds are made with an alcohol such as glycerol, a dibasic acid, such as maleic or phthalic acid, and an oil, such as castor, coconut, linseed, or soybean, or a fatty acid. Unlike natural resins, synthetic resins can be manufactured in large quantities and can be chemically tailored with great precision for particular uses. For example, the molecular structure of alkyd resins can be manipulated to vary their viscosity, their hardness, their solubility in water or other substances, and their capacity to mix successfully with various pigments.