Protection vs. Design Aesthetics
Let’s say you’re producing a print book with a cover on which you plan to print a heavy-coverage solid application of ink. Maybe you want to include type, a 4-color photo, and a background of a rich green mix bleeding on all four sides of the front and back book covers and spine. All of this ink, even after it has dried, will be prone to scratching. Your striking cover art could be ruined. This would be a good time to coat the press sheet with a flood application of UV coating, aqueous coating, a laminate, or even varnish—just to provide one more (transparent) layer of protection.
To clearly explain this, I want to take a moment to point out the difference between adding an additional coating, as noted above, and choosing a press sheet that is already coated. These are two separate things. Paper comes to the printer in the following forms: coated and uncoated. Uncoated paper is used for products like the text paper of a print book. Ink seeps into the paper fibers more or less depending on the hardness of the paper surface. In contrast, a coated sheet has been treated to have a far less permeable surface. Ink sits up on the surface of coated paper (this is called “hold-out”), whether gloss (shiny), matte or dull (far less reflective), or even silk or satin (various names for the coating in between gloss and matte or dull).
That said, the kinds of cover coatings noted above for print book covers (aqueous, UV, varnish, laminate) are applied on top of the printed press sheet. Almost always this is a coated press sheet (as described above). Think of this as an open-face sandwich, with the uncoated paper on the bottom, the initial paper surface coating on the next level, then the printed ink, and finally the overall aqueous (or other) coating.
Any time you “paint the sheet” (a printer’s term for laying down a heavy coverage of ink across the surface of the paper), it is prudent to cover everything with an additional coat of something.