urethane vs paint?

No matter what you put on it, if its not pressure treated, it will not last. So make sure it is. In general, oil based paint would protect it better. What are you using the plywood for?

The plywood is in place on a real old pickup camper. It's a damn good camper, but about worn out. I am trying to get a few morte years of use out of it.

The plywood forms the vertical walls of the camper, and replacing the wood would be impractical.

I'll put on some kinda surface sealer/treater, and hope it buys me some time. I'm just trying to find out what the better product might be. To further complicate my search, I've just been made aware of "water-based urethane acrylic" paint, which some people say is .....revolutionary?

Lee Carkenord

Anything with pigment in it will be better than anything clear for protection from the elements and UV.

My recommendation would be to go to a store that sells marine supplies and buy some Sikkens Cetol. It holds up very well on exterior wood trim on boats, and annually, you just scrub it with a scotchbrite pad and water to clean the surface, and then apply another coat or two. No sanding needed other than the initial sanding before you paint for the first time. The nice thing about Cetol is that it breathes. most paint failures are due to paint that doesn't breathe and let moisture escape. When the moisture can't get out, it causes the paint to blister & peel. Cetol doesn't have that problem.