I have never waterproofed the insides of the wood floats that i have built and have never had any of them leak in water yet, but there is always that possibility. I am wondering what most people use to water proof the wood on the inside just in case, that doesn't add alot of weight. I always completely glass my wood floats on the outsides.

Almost any sealer is good: thinned Epoxy, dope, balsarite, or polyurethane varnish, to name a few.

Hi Canada, I have recently asked this same question of some guys that have built many more models than I and they recommended that I mix up lots of thinned epoxy and pour it into the float through a hole and slosh it all around, though I have not yet done this, as with formers inside the float it will make getting any excess back out difficult at the very least, unless you considered this during construction and made the formers with large slots in the tops or the bottoms to allow the epoxy/dope to flow forward/back as needed until the inside is completely coated to your belief.