Exterior grade plywood has the same glue as marine, but your correct marine grade has no voids, I just did mine, I used exterior grade 1/2" ply with a thick coat of polyester resin. Cheaper than epoxy and locally available. Take care with your edges since there will likely be voids. I also had to replace one entire stringer, repair another stringer, and re-glass all three stringers. Be sure to check everything out now that your in there.