Tell me what you know about....

[Funrover]

Marine grade paints     I was going to go with marine wood...but that is $$$$   so if I use normal(in the non marine sense) wood with marine grade paint as a roof will that work just as well?

[Old Goat]

Marine grade paints are nothing more than  industrial grade paint being marketed
with a marine name so bigger $$$ can be charged for it..There are plenty of good urethane based industrial exterior paints around for a much cheaper price that will last equally as well as the higher priced "marine" paint. Rust o leum's Industrial  and Benjamin Moore's Silathane are two of them. AC grade plywood will work fine for your project....

[wavery]

I don't think that you will want to put any kind of automotive or marine paint on plywood. These paints are very hard and brittle. They are designed to go on hard, non-fibrous, surfaces like metal or fiberglass. The plywood fibers are continually moving with vibration and/or stress. That will cause the paint to crack, split and peel.

You may want to look into an elastomaric paint. It is more like rubber. It's very thick and designed to seal water out. Of course, the plywood must be primed first.
http://www.dunnedwards.com/retail/content.asp?Category=75&Content=0
Put the word "Elastomeric" in the search box. Then click on any one of the selections for information about elastomeric paints.

You may want to look into purchasing some pre-coated sheets of aluminum to cover your plywood. Uncovered plywood may not be the best medium for a PU top.

[mike4947]

If your talking about using plywood as the main roof surface please remember you are replacing a VERY thin sheet of aluminum, styrefoam and thin ceiling paneling bonded together for strength. There is little weight in that and the lift system isn't designed to lift much more. PU lift systems are only rated for the actual roof weight + 150 pounds.

[Funrover]

Thanks everyone!    I understand the weight part!!! I am thinking it over still!