I have used several products, and have used one of the franchise Tank Renu services. The best and easiest solution to a rust or leaky tank is Caswell Gas Tank Sealer. It is an epoxy you mix and pour in the tank. I have used it to seal tanks that had literally hundreds of pinholes and even 1/4' gaps by backing the holes up with masking tape. I have used it to fix failed applications of other products. As long as you can get the inside of the tank relatively clean and dry it works. The stuff is the consistency of gear oil and is crystal clear. You pour enough in to coat the inside of the tank and move it around to make sure it gets everywhere. Cures hard as a rock and is impervious to chemicals. You can use it to pot electronics too, and as grout to set heavy equipment. No relation, just very impressed.

Good to know....