I have always used phosphoric acid. It does a really good job eating away the rust and it doesn't harm the base metal, its not nearly as dangerous as muriatic acid but you still need to be careful with it. The tank in the pic I posted took about 3 hours of soaking in acid (rotating every so often) to thoroughly clean and it was really nasty. Acid clean, rinse with water, dry thoroughly with forced air and then Por15. So far I'm really impressed with their products. One of the lined tanks is on my track bike that sees race fuel all the time and it still looks great.