Varnish is essentially ink without pigment. It requires its own printing unit on press. It can be wet-trapped (printed in-line at the same time other inks are laid down), or dry-trapped (run as an additional pass through the press after the initial ink coating has dried). The latter often provides a glossier finish. Varnish comes in gloss, dull, and satin (in-between dull and gloss), and can be tinted by adding pigment to the varnish. From an artistic standpoint, you can play a dull-varnished portion of the sheet against a portion without varnish or with a gloss varnish. This contrast can give emphasis to certain areas and/or give the impression of depth.
UV Coating is a clear liquid spread over the paper like ink and then cured instantly with ultraviolet light. It can be a gloss or dull coating, and can be used as a spot covering to accent a particular image on the sheet or as an overall (flood) coating. UV coating gives more protection and sheen than either varnish or aqueous coating. Since it is cured with light and not heat, no solvents enter the atmosphere. However, it is more difficult to recycle than the other coatings. UV coating is applied as a separate finishing operation as a flood coating or (applied by screen printing) as a spot coating. Keep in mind that this thick coating may crack when scored or folded.
Aqueous coating is more environmentally friendly than UV coating because it is water based. It has better hold-out than varnish (it does not seep into the press sheet) and does not crack or scuff easily. Aqueous does, however, cost twice as much as varnish. Since it is applied by an aqueous coating tower at the delivery end of the press, one can only lay down a flood aqueous coating, not a localized “spot” aqueous coating. Aqueous comes in gloss, dull, and satin.
Laminates come in two types: film and liquid, and can have a gloss or matte finish. As their name suggests, in one case a clear plastic film is laid down over the sheet of paper, and in the other case a clear liquid is spread over the sheet and dries (or cures) like a varnish. Laminates protect the sheet from water (including perspiration from the hands) and are therefore good for coating items like menus and book covers. For more money, one can even specify a porous, lay-flat laminate (the interior of which is covered with numerous “V”-shaped cuts in the plastic that minimize the “curl” one often sees on paperback book covers due to moisture seeping into the uncoated side and causing it to expand). Laminates are slow to apply and costly but provide a strong, washable surface. They are the superior choice for protecting loose books in transit.