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Hands down the Spar. Polyurethane would crack letting water under it and then soon fail because it is a interior finish intended to be harder and more scratch resistant. Cabot makes a pretty good spar varnish but if you are looking for the best I would recommend using Epifanes marine grade spar varnish. It is formulated for the deck of a boat. You would find it at places that sell boat supplies.What would give me the best finish on marine plywood in direct sun. Spar varnish or polyurethane? Is there something else on the market that would work better. Can not use epoxy do to health problems. This is going in a boat for the floor and the sides.:boat: