solutions formed by the combination of vegetable oils and natural or synthetic resins in organic solvents. Oil varnishes are prepared from oils with sufficiently good drying properties that have been refined and polymerized (to a given viscosity). Natural resins used for this purpose include rosin and, less frequently, amber and copal; the common synthetic resins include phenol-aldehyde resins and various products of their modification. Certain oil varnishes are manufactured using natural or synthetic bitumens. White spirit, solvent naphtha, gasoline, turpentine, and xylene serve as solvents for oil varnishes. In addition to the basic components, oil varnishes also contain desiccants (salts of cobalt, manganese, and lead) and certain other secondary additives.
Oil varnishes are prepared according to various methods. In some cases the resin is first melted, oil is added, and the mixture is boiled down to the desired viscosity; in others the resin is added to the heated oil. A solvent and desiccant are subsequently blended into the varnish base produced by these processes. Oil varnishes are applied in the same manner as paints and lacquers.
Oil varnishes are classified as short-oil (0.5-1.5 parts oil to 1 part resin), medium-oil (1.5-2.5 parts oil), and long-oil (2.5-5.0 parts oil), depending on the oil content. Long-oil varnishes form highly elastic and weatherproof films upon drying; short-oil types produce films that are harder and more lustrous, yet less resistant to weather conditions.
Oil varnishes are used to coat metals and wood laminates and to impregnate electrical windings. They also serve as binders in the preparation of pigmented paints and varnishes—for example, primers, putty, and enamel paints, or oil enamels. The last are used in the manufacture of weatherproof coatings, as anticorrosion coatings for metals, and for various other purposes. The manufacture of oil varnishes is currently being gradually reduced because of the increasing use of alkyd varnishes, which have a lower oil content and form coatings with higher performance properties.