I asked around a lot and the most commonly accepted answers were polycryllic, polyurethane, and expoxy paints/finishes.
This guy has some cage building plans available and discusses the specific brands he uses.
I used polycryllic on my rack but it's new so I have no idea how well it will hold up. I would say you'll need at least 3 coats and probably, though if you use that. For a cage, I would go with something else because I felt like it took a lot of the stuff to even begin to form a seal over the plywood. I think I'd try polyurethane next time. See the link in the paragraph above for a recommended variety which seems to be a widely tested product.
The epoxy seems expensive but is durable enough to turn plywood into a fish tank, so it will certainly waterproof it enough even for high humidity species. I found a good article discussing
brands here. If it's working for fish it's certainly safe enough for a reptile and possibly even amphibians.
Good luck and I hope someone with actual experience can chime in, but hopefully that will help you get started.
Something different.
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