In the production of printing ink, varnish can refer to either a combination of oils, resins, waxes, solvents and other materials used as an ink vehicle. The use of varnish tends to increase the gloss of a printed ink.
The term varnish also refers to an overcoating applied to a printed piece following printing, performed on or off the press. Varnish is typically added to a finished printed piece either for aesthetic reasons (i.e., to increase gloss and provide a better overall look) or to protect the printing from wear and tear due to handling or contact with moisture or chemicals. Varnish that is applied to an entire printed surface after printing is called overprint varnish; varnish that is applied only to certain portions of a printed surface, primarily for aesthetic reasons (as on book covers) is called spot varnish. If the varnish is added as an overcoating after printing, it is important to ensure that the formulation of the varnish is compatible with that of the ink, or bleeding and other printing defects can occur.