Marine-grade plywood differs from ordinary-grade plywood or external-grade plywood in a number of ways. Marine plywood usually contains a larger number of plies, or layers, per quarter of an inch thickness of the wood, and fewer knot holes than either of the other two types of plywood. The plies of marine plywood are held together with waterproof glue. Marine plywood needs to be treated in order to avoid the wood getting waterlogged and rotting. Treating is the process of putting a protective layer or layers on it to prevent the wood from being damaged or suffering decay from fungus, insects or moisture. There are several steps to treating marine plywood in order to improve its durability and protect it from the elements and create an even layer for paint to go on smoothly with no unevenness of absorption.
Place the first plywood on the sawhorses. Examine the plywood carefully with your hands to locate any raised knots or other irregularities in the wood. Sand them flat with the coarse paper either by hand or using the electric sander.
Sand the whole of the plywood lightly on both sides and along the edges to smooth irregularities and give the latex paint something to cling to.
Stir the latex paint well. Test a small portion on the wood with your brush. If it seems too thick or is not going on in one smooth stroke, thin the paint slightly with the marine latex paint thinner. Test again until it reaches the desired consistency, which should be that it gives good coverage over and into any small imperfections or the grain of the wood.
Paint the first side of the plywood in small areas, concentrating on getting a thin, even coat with good coverage.
Let the plywood sheet dry for at least a day.
Turn over the plywood and repeat Step 4. Once you have finishing treating the top side of the plywood, carefully treat the edges.
Allow the plywood to dry for a day before touching and at least a week before adding epoxy varnish to seal the wood completely.
Repeat Steps 4 through 7 with each additional sheet of plywood you might wish to treat.