Lining a tank

My chopper's tank leaked when it was new. Bike builder replaced it 3 times before giving up. He suggested sealing it. The tank was sent out to a shop that was going to put some sort of sealant on the inside. Keep in mind that the tank already had fuel in it. Well, they did the acid bath deal and then the sealed it. A few months later I noticed there were several spots that the sealant was missing. I went and bitched at the builder who just sent me over to the other shop. They told me that there particular sealant will only adhere to a new, unused tank. There was nothing they could do about the pealing. Be careful of what a salesman tells you......