What is the difference between marine plywood and treated plywood and will paint stick to treated plywood?

in addition to renegade's excellent answer, what distinguishes marine grade from exterior grade is the absence of interior voids. Interior voids leave the lumberyard with enough atmospheric moisture already trapped inside to ensure rot from the inside within 10 or 20 years. EXT grade ply can have some maximum number of interior voids (a knothole in an interior ply) of a specified size (don't remember right offhand), but it *is* made with waterproof glue. far as I know, treated plywood is treated against insects, not rot. I have had excellent results using B/C EXT ply saturated in polyester resin and covered with a layer of fiberglass cloth for floors and decks in my boat. If you need less than a complete sheet of plywood, you can save money on the marine grade by buying 1/2 the thickness you wanted, and laminating two pieces together with a marine grade epoxy.