I''m a little confused. You said that your boat has 3 layers of 5/16 ply (total 15/16?) or do you mean a three ply 5/16 inch plywood. You said this was cloth covered. Is this a wooden boat? If so go to the Wooden Boat web sight (www.woodenboat.com)for advice. If you are down to plywood on the deck don''t use polyurethane. Put a layer of Dynel set in epoxy
. This is what I would have thought the West System guys would have said. This makes me think that I do not have a true picture of what you have.