I tried that phosphoric acid on a pan muffler I had. Got the pure acid at Meijer in the pool cleaning section. It smoked like hell and oozed this green toxic waste looking stuff, but it sure cleaned my muffler. But the guy at the store warned me. Every second the metal touches air after this acid reacts with it will begin to rust it. So you have to paint it immediately.
My point being, after you clean your tank out with whatever it might be a good idea to put some kind of gas proof paint on it to insure that it doesn