On Tuesday, December 18, 2012 4:45:28 AM UTC-8, Greg Guarino wrote:
Context:Plywood built-in bookcases. I assume I should seal all surfaces in some way, even though the top, bottom, back and one side will never be seen. True? If so, what should I use? If it simply doesn't matter, I'll probably try to use up some cans of whatever is on hand.

FYI, no profesional cabinet shop is finishing unseen faces. Ply won't warp like solid wood. Really not necessary unless you have some extreme condition backed against a slurry wall or some other severe moisture situation.