Oil paints can be sucessfully thinned using oil-based varnishes, varnishes made with oil resin or resin-based varnishes. Natural varnishes contain drying or semi-drying oil and natural resins. The properties of the resins and oils used determine the effect that the varnish has on the paint that requires thinning. The addition of soft resins gives the film of varnish more elasticity and a certain matte appearance. If hard resins (such as copal, amber, and rosin) are used, the film becomes harder and more glossy. At one time, varnishes made purely of resin, such as mastic or pine varnish, were used quite extensively for thinning. Resin varnishes are usually quite thick and have a resin content of up to 30%, dissolved in pinene.
In the last quarter of the 20th century, varnishes based on synthetic resins, more often than not acrylic ones (such as acrylic-pistachio varnish) gained in popularity. Current acrylic-pistachio varnishes consist of polybutyl methacrylate and pistachio resins (22-25%), and butyl alcohol is used as a solvent, with a small amount of white spirit added (about 2%). This varnish is quite thick, so it is usually diluted to a ratio of 1:1.5 or 1:2. The varnish gives the paint film greater elasticity and weather resistance.
A large number of oil-acrylic varnishes are available, some of which are fairly well-suited to the task of diluting the oil paints used by artists. Varnishes containing ketone or acrylic resin (15-25%), white spirit and poppyseed oil are used. When mixed with oil paints, they enhance the degree of application to the oil base, and increase elasticity and brightness, giving the work a consistent sheen. The technique involves mixing them into the paint, whereupon they limit the process of yellowing. Thanks to the poppyseed oil they contain, these varnishes are versatile and can be used for a variety of purposes. The varnish dries more quickly than the oil but more slowly than an additive, despite having the same properties.
