Re: plywood type for fiberglass .
I had thought that most people knew that the 'X' in ACX was exterior but perhaps not. It was my assumption that the OP was talking about exterior grade.
In terms of polyester resin I can't agree with that. There are too many problems. Polyester is not an adhesive. To get maximum strength you need to put on additional layers while the first layer is still tacky. But even if you do it won't stick to the plywood. I would use epoxy. Epoxy is an adhesive and will stick to both the original fiberglass and the new wood. Epoxy is also waterproof whereas polyester is only water resistant. You have to make polyester thick to have a reliably water tight boat. For example the glass on my boat is about 1/4" thick with polyester resin but all the repairs I did were done with epoxy.
I've heard of people building plywood boats with zinc coated screws and then having these screws rust in salt water in one season under a thin layer of polyester glass. It's something to consider.
1980 Southcoast Boats Explorer #34
Copy of International 21.
No connection to South Coast Seacraft.