I coat lightweight spackle patches with "Future" no-wax floor coating. In my experience it's bulletproof, even on seaplanes.
I think it's an acrylic; it can be thinned with household ammonia. I put it on with a foam brush but I've heard of people thinning it with Windex and using it in an airbrush. I coated my Seawind with it before it's first flight to add a little durability to the foam's white finish, and after a season of flying the Future finish is still in good shape.