Phosphoric acid is what is used in most tank coating "kits". It's slower but much more user friendly than muriatic acid. I used muriatic because it's available locally & dirt cheap. As noted by 4ontheFloor your tank will start rusting soon after treatment no matter what acid you use for the cleaning. Unfortunately keeping it filled with gas won't stop the rust because you can't keep it full. As you use the gas the level falls bringing moisture laden air into the tank. If you use an acid cleaner you'll need to coat the tank. You shouldn't have any problems with the coating sticking IF YOU FOLLOW INSTRUCTIONS. I've done several and have never had a problem. The coating I prefer is Red-Kote. I buy mine from the local Bumper to Bumper jobber but you can get it on line too.

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