Originally Posted by duct tape
Digging up an old post for a new question.
I'm luke warm on the Polymax having tried it for something else on my raft. Now I'm outfitting a cat frame and need floors for two bays. Where can I get real marin grade plywood? The stuff at Home Depot is not true marine grade and looks cheap. Everything online is near an ocean (I'm not) for boat building.
Or maybe there's some other synthetic option which is as stiff for carrying loads and walking on as wood?
Lastly, best paint for wood? Or spar varnish?
You usually have to order marine ply and really for a raft floor it's entirely unnecessary. I researched marine ply extensively for a jet boat project that I eventually mothballed, but in the end marine ply is really important in high stress, marine environments - or if appearance is paramount. The most common non sailing or non motorized used is in driftboats, where the wood is shaped, prestressed and is the structure of the craft (failure would be catastrophic). For decks, dryboxes and floors in rafts it is a complete waste of money unless you want the bright varnished finish look of high quality hardwood ply.
A vary popular option for guys building painted boats is MDO (or other acronyms) for medium density overlay - its a fir based high laminate count, low void plywood typically used for street signs. It's covered in brown paper so a clear coat is nothing special but it's really good stuff and half the cost of marine and readily available at most real lumber yards (not home depot). The other option is simply ACX - "A" sanded and plugged finished side - "C" non plugged sanded side (with knots) - X - exterior glue - equivalent, in most cases to that used in marine. The issue with ACX is since it's fir, it checks and is not low void (or void free like marine) - so structurally it can be hit or miss (again in high stress environments).
Oh and virtually all marine ply is imported. There area a very few high dollar custom manufacturing joints out there but I doubt we could even locate a sheet from them. There is also a huge sustainability issue with hardwood plys - many come from tropical hardwoods some managed harvests, some not so much.
I bring this up, again, because there are many folks that talk about marine plywood here that it just makes me wonder, if everyone is made of money? Do some research elsewhere and see what you find on the subject, but in my opinion, it's complete overkill or a waste of money.
In the end, if it were me I'd drive right passed home cheapot and find a real lumbar yard and get some MDO. Paint it, add some very, very fine grit to the last coat and be happy. Recoat occasionally with paint with grip in it.