Its always a problem sealing timber tops as the timber moves under your covering. The old method was to cover the top in canvas laid upon wet paint. You could always lay woven rovings on to epoxy but I would suggest laying a plywood deck first then sheath the deck in woven rovings. Make sure the timber is sound before you do anything - boric acid mixed with borax dissolved in antifreeze will stop any rot. Look up chemotherapy for boats if you'd like more info.
