A liquid shellac or plastic-like coating put on a printed piece like a layer of clear ink, varnish adds a glossy, satin, or dull (matte) finish. Part of the printing or finishing process, varnish is applied like a final layer of ink after a piece is printed. It may be clear or tinted.
Varnish can be used functionally to reduce glare or enhance readability or as a design element to smooth, highlight, add texture, or create added dimension. It can be a flood coating (applied to the entire page) or a spot (applied to only parts of the page or design).
The term varnish is often used informally to refer to other types of finish coatings including UV (ultra-violet) and AQ (Aqueous, water-based). Discuss the use of a varnish or other finish with your printer to be sure you are in agreement on the actual type of treatment for your specific job.
