What is Varnish?
There are two principal types of varnishes: spirit varnishes and oleoresinous varnishes. Spirit varnishes are solutions of resinous materials in volatile solvents, such as alcohol and ether. They are usually made by stirring or agitating the resin in the solvent until the resin dissolves. Heat and additives are sometimes used to speed the process. The mixture is then refined and thinned to working consistency. Spirit varnishes dry by evaporation of the solvent. Shellac, lacquer, pyroxylin, dammar, and japan are spirit varnishes.
Varnish is a glossy coating that is applied as a liquid and dries to a hard clear or semitransparent film. Varnishes provide decorative and protective coatings for a wide variety of surfaces. They are used extensively on wood because they do not obscure the grain. They are also used to line metal food containers, to make paper resistant to moisture, and to insulate wire and cable.
Oleoresinous varnishes are mixtures of resinous materials and drying oils, such as linseed oil and tung oil. These varnishes are usually made by heating the oil and resin mixture to temperatures ranging from about 450° F. to 600° F. (280° C. to 400° C.). The mixture is then cooled and thinned with a solvent, such as turpentine or petroleum naphtha. Oleoresinous varnishes dry by evaporation of the solvent, by oxidation, or by polymerization, a chemical reaction in the drying oil. Driers consisting of compounds of lead, manganese, and cobalt are usually added to speed the drying and hardening process. The resins used include rosin and synthetics of the phenolic, alkyd, epoxy, and polyurethane types. The urethane varnishes are the hardest and longest-lasting.
So-called "short" oleoresinous varnishes, made with about 5 to 15 gallons (20-60 liters) of drying oils for each 100 pounds (45 kg) of resin, are mostly quick-drying and produce hard glossy films that are easily sanded. They are used mostly to finish furniture and interior surfaces. "Long" varnishes, which have a greater ratio of oil to resin than short varnishes, generally dry more slowly and produce more elastic coatings. They withstand weathering and are water-resist ant, making them ideal for exterior use.