The acid will leave a coating that will resist rust for a couple of years. If no water gets in then the gasoline will keep it from rusting.
I have to disagree. The only coating phosphoric acid leaves behind is surface rust once the moisture in the air hits the surface. Gasoline will do nothing to stop the rust unless you never drive the truck. In a vented system, as the fuel is used it's replaced by air, moisture laden air. The moisture will cause the unprotected metal to rust. I've used phosphoric acid to de-rust gas tanks and I've used muriatic acid to de-rust gas tanks. Once the rust & crud are gone you must use a coating if you want the rust to stay away. I've used several and my personal preference is Red-Kote. I once read a post that muriatic acid will cause the metal in a gas tank to change properties and become brittle. I can't call that person a liar BUT I used muriatic on my '38's tank five years ago and have had NO issues since.
