I've used pressure treated plywood successfully but only after drying it out completely by sitting the plywood behind our woodstove. I then treated the plywood with a slow cure penetrating epoxy to eliminate rotting issues.

However marine grade plywood is certainly a better but at a much greater cost.

Even better then marine grade plywood would be coosa board.

Thanks guys. I was really hoping not to have to take it out but guess I gotta bite the bullet.

Thanks for the link!

So after I get the plywood peeled off I mix some epoxy resin and pour it in the shell then lay the ply in that? Is that right? Next I would need to resin the top WITH cloth or no? What would be good to clean the shell with after I get the ply scrapped out so the new resin and ply stick? This is the tank cover / seat support so it gets a LOT of use sure don't want the ply coming loose. Thanks!

I would hot coat the plywood and used thickened resin on the shell. I used a notched trowel to spread the resin on my hatches. I also used pieces of plywood rather than a full sheet. I cut my plywood to fit then I cut the piece into smaller pieces and left a little bit of space in between each piece. Then I forced more thickened resin in the joints and glassed over the top. I was very happy with the strength, we'll see how it holds up.

What is 'hot coat' the plywood? Thanks

Started the floor repair. It sure was shot. Gonna get Marine ply this week sometime after work and lay it it with resin I hope.

I don't know what the "proper" fix is, but when I re-did my floor after I put the plywood down I used a thin woven sheet of fiberglass cloth on top of the wood...that way the wood is permanently sealed so no water can rot it out again... just a thought

A couple more tips these guys taught me when I did mine was,

cut the holes out after everything is all said and done. And then coat the edges .

Drill some holes in the plywood to let the resin find a place to go instead off squeezing out the edges.

Put some Saran Wrap or some kind of plastic between the access holes and your work table so you don't glue your hatch to the work bench!

Oh and get lots of heavy things ready before hand to set on top of the ply., sand bags, tool boxes cinder blocks. And make sure your work table is perfectly flat, floor works well too

Yes I agree I have the cloth for that.