Mike,
I agree with Phill, when we built our boat we used two coats of Timber Tough resin (a thin Epoxy resin) that will sock into the wood before we painted the boat. We used 3mm ply foredecks on our Mossie and it cracked within the first season (1996). After talking with Jim Boyer he advised us to remove the paint and add 2oz cloth, wet it out and clear finish paint or paint to protect the resin from UV. The extra boat weight was not noticeable and we have not had to do anything to it since.
Regards
Peter