There are a number of commercial products designed to create a waterproof coating on the inside of your frame Weigle Framesaver is one, and I have also heard of using boiled linseed oil, Boeshield and WD40 for this purpose. I myself have used both Boeshield and WD40. the idea is to spray the inside of the frame using pinholes and other larger holes like bottom bracket, seat post etc and rotate the bike around giving the liquid a chance to coat the inside. In a couple days it will have dried and be a hard gummy surface on the inside of the frame. Boeshield is a spray lube apparently invented by the Boeing people. No way this can hurt your frame excepting you should never do this to a Brompton seat tube. Once convinced it is dry you can also plug the little holes in the chainstays, seat stays and fork with beeswax if you like. Once warmed in your fingers it is malleable and doesn't come out.