I'm planning a project (home built small off road camping trailer) and considering finishing the exterior plywood with "poor man's fiberglass," which typically involves painting/bonding canvas to the surface using either Titebond II or latex exterior house paint. I'm wondering whether I can enhance durability by using marine paint with the canvas -- either a one part poly or an alkyd enamel. Any thoughts?

No, you want a flexible paint. Alkyd marine paint is rigid unless you really thin it. Thinned latex or acrylic is what you want. Don't over-saturate or you'll get cracking. You should make sure the canvas is pre-shrunk as well.

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SWCA #3356, SWHF#611

Do a search and see what they use to paint canvas covered canoe's

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Don

I was thinking a coat of the "X" primer that adheres to anything-followed by bed liner. I used bed liner in the bottom of a fiber glass kitchen sink that got a lot of hard use-it never failed.

You could use marine plywood, or saturate the surfaces with fiberglass resin prior to painting. Or you could have pickup truck bedliner sprayed on the surface.