For water proofing you'll need some sort of epoxy. There are epoxy paints and there are pure epoxies. Either will work but it sounds like you'll want a paint. Sweetwater makes epoxy paints for aquaria that are safe to use around animals although you'll need to wear a respirator while applying it due to the solvent carriers.
Some of the garage floor paints will also work but you'll probably have to tint these yourself to get a black color.
Of these only the solvent based types will waterproof the cage.
The pure epoxies are clear so you would have to apply those over a black paint of your choice. The bar top epoxies work well for this.
Another choice is to use Drylok's Latex Masonry Waterproofer. This is a hydraulic cement based paint that has been used to waterproof wood even though it is not designed for such an application. It is a highly textured paint (like cement) and would make for a hard to clean surface. A friend of mine used this is plywood cages and then top coated with several coats of a high gloss paint and he says it has made for a cheap but somewhat easy to clean surface. I just got off the phone with him and he says the cages are still holding up. I think it's been about a year.
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Currently keeping a small collection of various Gonyosoma. Both G. janseni and G. oxycephala.