What kind of water sealer can I add to semi-gloss paint for a water proof job?

I'm not sure what you are doing with your paint....but I know you can put Polycrylic water based sealer on top of a latex semi-gloss paint to make it water proof. I faux finished my bathroom countertop with latex paint and sealed it with the polycrylic. After it dries for a day or so, it's totally waterproof. I heard you can use any thing on it to clean it too, but just not something that's a solvent. I put several coats of it on, and sanded with wet dry 220 grit sandpaper between coats. If you do this, use some bar soap on your sandpaper, this keeps it from sanding too deep. The last sanding before the last coat, use 320 grit for an extra smooth finish. Good luck!