We're going to try building a cob bathtub, and I was wondering if anyone has any ideas for the best way to waterproof it. We framed the rough shape of the tub with wood, and plan on cobbing the whole thing soon. I read of a lady who sealed a tub with hemp oil and had good results, but problems with leaking around the drain forced her to take it down. We plan on using hemp oil a lot in the house, but I'm not sure if it's THAT water resistant. We'd rather not have to reapply the oil every six months or something like that, also.. I was wondering if a lime plaster type of finish might be the way to go?