What is varnish and when should I use it?

Varnish or aqueous coating is best described as ink without pigment. These coatings can be applied in a gloss, dull or satin finish. Varnish can be used to highlight printed images for effect and to offer some protection from smudging, scuffing and scratching. Note: 

You cannot ink-jet print, glue or foil stamp over varnish, so spot varnishing will leave an uncoated area for these applications.