I make many fountains and use the basic recipe of equal parts of peat moss, perlite and portland cement. To waterproof, I first use a sealer like Thompson's water sealer (on finished project, of course). After that is dry, I paint the fountain bowl with a gray waterproofer paint (like you would use in a basement). When that is dry, I cover with a thin layer of hydraulic cement (a cement that waterproofs and dries within minutes). Fountain bowls take a lot of water, so I've found this 3-step process works the best for me. When I just make birdbaths, I just use the concrete waterproof sealer, because the amount of water isn't so great. Good luck!