Polyester or Vinylester resins do not adhere to plywood all that well. Epoxy on the other hand, as suggested above, is an excellent adhesive. To get anything to stick the old paint will need to come off as also mentioned above.
If you don't plan to walk on it consider just using epoxy resin w no glass, a couple of coats will do, scratch it and then prime and paint. This will give you watertight and longevity, the glass is unnecessary to meet those needs. Epoxy is very flexible, a great adhesive and an excellent vapor barrier.
Glass really doesn't bring much to this type project. With Polyester/Vinylester it may even contribute to delamination from the plywood do to flexing and expansion/contraction from temp changes.
Both Polyester/Vinylester and Epoxy do not tolerate UV, and none will readily accept paint without a light sanding and a prime before paint, so scratch/prime and paint for sure.
$0.02