Fake rock background?

[TheOMan]

I'm making a fake rock background out of expanding foam, paint, and grout for my axolotl tank. Unfortunately the expanding foam is toxic to aquatic life. Am I right in thinking that sealing the whole thing with a few coats of a pond waterproofing product like pond seal would stop any chemicals leeching out into the water and poisoning my axolotl? Or do I need to rip the whole thing out and start again with ab aquarium safe product?