Varnish is a finish applied to wood or other surfaces in order to provide a clear, hard, durable, protective finish. The materials used for varnishing are called drying oils. There are many types of drying oils, both natural and synthetic. Walnut oil and linseed oil are most often used in violin varnish in combination with amber, copal, rosin or other resins. The oil is prepared by cooking or exposing to air and sunlight. The resin is"run" by cooking and reducing it's weight and concentrating it's color. The thickened oil and prepared resin are then cooked together and thinned with turpentine into a brushable solution. Oil varnish drying time depends on the ratios of oil to resin and turpentine and may be sped up by exposure to UV light. Varnish finishes are often, although not always, glossy. As opposed to paint, which contains pigment, and is opaque, varnish has little or no color, and is transparent.

After being applied, varnishes harden either by evaporation of a solvent, or by a chemical reaction.

Most natural varnishes consist of a gum or resin dissolved in a solvent. Depending on the solvent used, the three main types of natural varnish are spirit varnish (which uses alcohol as a solvent), turpentine varnish, and oil varnish. Spar varnish (also called marine varnish) is high quality waterproof and sunlight resistant varnish named for its use on the masts of ships (called spars). The word lacquer refers to quick-drying, solvent-based, varnishes or paints. Lacquer is also a very durable, slow-hardening, varnish obtained from the sap of a varnish tree. Shellac uses alcohol as a solvent. Although not a particularly durable varnish, shellac is sometimes used as a primer for paint or varnish.

Synthetic varnishes include urethane or polyurethane varnish, varnishes that use water as a solvent, and epoxy varnishes.

See also desert varnish