varnish - 1) hard, varnish-like coating formed from oil oxidation products, that bakes on to pistons during high-temperature operation of automotive engines and industrial machinery. Varnish can accelerate cylinder wear. Varnish formation can be reduced with the use of a detergent-dispersant and an oxidation inhibitor in the oil. See engine deposits. 2) In ink formation, varnish - composed of resin, solvent, and additives - is the vehicle to which pigment is added to make printing ink.