Varnishes for Oil Painting
Varnishes are designed to protect an oil painting from the dirt, dust, and grime prevalent in air. Over time, any painting is exposed to pollutants that become trapped in the surface of the work. To clean the artwork is impossible unless the piece was properly varnished upon completion.
Varnishes are removable by definition, otherwise we call them topcoats. The proper way to clean an oil painting involves the removal of dirty varnish - so a topcoat will not suffice.
Most oil varnishes are damar resin which has been dissolved in turpentine. Damar varnish provides a thick, strong, and ultra glossy finish. Though damar is an excellent and traditional varnish, it is prone to yellow and darken over time. Modern synthetic varnishes are less prone to yellowing or darkening, and also come in matte and satin finishes.
Oil paintings should be allowed to dry on average 6 months before varnish is applied, or the varnish will crack as the paint oxidizes through it.