well, the fiber glass itself is not waterproof. in facts, FG is just the material, its like some ripped glass and in my case, ripped glass put into a sort of cloth. you must put some sort of resin to it so it chimically becomes solid. epoxy resin is 100% waterproof so we use, or at least I used epoxy resin with my fiberglass. you could only use epoxy resin, but the fiber makes the wood stiffer. only use fiberglass if you build a real big tank. i think i could of use only resin or other sealant like gel coat or liquid rubber. the thing is, i wanted to build some experience with fiberglass since i plan on larger projects in a near future.