Varnish is a finish which can be applied to wood and other surfaces to create a hard, glossy film which will resist the elements. This type of finish is used in a wide range of situations, and it is a very popular method of finishing for floors, boat trim, and some cabinetry. Many home supply and hardware stores stock an array of varnishes for people to choose from, ranging from high-rated marine varnish which will last for years to more delicate furniture finishing varnishes.
The goal of varnishing is to protect the underlying surface from damage. The finish will resist scratches, water, ultraviolet radiation, spills, chemicals, and many other hazards. Over time, the varnish itself may discolor, flake, peel, or crack, but the underlying surface will be perfectly safe. Eventually, it may become necessary to strip the finish, sand the surface to condition it, and then reapply a new layer.
People have been using varnish since the days of the Ancient Greeks; the term “varnish” comes from “Berenice,” a city in modern-day Libya where this finish was supposedly first invented. Varnishes contain resins suspended in a solvent and blended what is known as a drying oil. Drying oils are oils which will firm and become glossy when they are exposed to the air. Linseed oil is a classic example of a drying oil. The presence of the oil requires a curing time after varnish is applied to give the material a chance to set all the way.
People can use multiple layers of varnish, in a process known as lacquering. This results in a finish with a very high gloss and a high level of hardness. It is also possible to use fewer layers for a less slick finish, and some varnishes have dullers for a semi-gloss appearance, for people who dislike the classic high shine of traditional varnishing. In all cases, the finishing material is transparent and minimally colored, if at all.
In addition to natural varnishes made with material like shellac, it is also possible to find polyurethane and acrylic varnishes. Purists sometimes insist on using a natural varnish, especially for furniture and home restoration, so that the new finish is as similar as possible to the old, while other people find synthetic varnishes much more durable and long lasting, and therefore preferable. It is always a good idea to spot test varnish on a hidden area of a project to see if the finish causes unsightly staining, and to allow varnish to cure completely between coats to avoid creating a tacky or gummy finish.