I would say to veneer a piece of metal over the 3/4" plywood and paint the metal or fiberglass the 3/4" plywood & paint/gelcoat. We are currently sitting in NM with a with a food cart that is sheeted with plywood on 3 sides (2 small side sections and the full front over the tongue). The plywood was added in December 2009. Painted (4 coats) Ace Exterior Gloss Latex house Paint in F41 "Decadent Blue". In 2011, we rolled another coat of the Blue on to touch it up from traveling. In the summer of 2011, we rolled Rustoleum Hammered Copper on it trying to keep the plywood from cracking any more from the sun. The Hammered is great for bug removal. We just finished painting it again (changed menu to ice cream, now it's PASTEL
). We certainly don't intend the cart to last as long as our bus PLUS we can remove the plywood with only minor difficulty. Can you say the same? Because you will be replacing the plywood in a few years due to sun damage. If you insist on plywood try to buy the marine grade plywood. It will hold up very well to moisture... much better than pressure treated which isn't treated very well now-a-days.
We cleaned and resealed all our fixed glass windows/windshield with elastometric sealant (Henry's 212 Clear
). We filled the spaces we removed windows from with plywood covered with a heavyish aluminum flashing. We got lucky and found a place that carried flashing that was just wide enough to cover the plywood as long as we reused the original window frames. But that was what WE did. Others have done differently. I'm sure they will chime in. There is no one "right" way. Read what others have done and find what you think will suit your needs (and pocketbook) best.
On edit: plywood quality really sucks and don't get me started on #1 wood... "premium" my foot!