So glad I found this page, I've been trialing using a plaster for casting candle containers and have exactly this issue. I was going to try waterproof PVA (I'm in the UK so I think that's modgepodge in the US). I'm using coconut wax which has a low melt point so the result is total discolouration as it melts. I shall update you on the trial! I'd post a pic but I can't work out how to do that....