Haven't been around the forum in quite a while and was checking out old threads.
The best chemical I've found for cleaning rusted fuel tanks is Oxalic Acid. It is a naturally occurring acid and is among the group 'Weak Acids' as opposed to strong, caustic & corrosive acids. It's much safer, inexpensive and the mixture once mixed up can be used again & again. It not only cleans fuel tanks very nicely, but your 'bolt bucket' of old rusty fasteners you refuse to throw away, and old rusty mechanical parts clean up to fresh new metal!
It's safe enough to be used in beekeeping, spraying the hives to kill a nasty mite infestation. It can also be used on boat decking, wood members & brightwork & fasteners to remove stains. It's also used in weak concentrations to remove carpet stains or rust or blood stains on clothing and is found as component in various cleaning products. Won't hurt chrome but will remove the rust.
For fuel tank cleaning you use 8 to 16 tablespoons of the crystals per a 5 gallon tank, preferably mixed with warm or hot water, but cold's okay. Fill fuel tank with the mix, or mix in place in tank and let sit at least overnight. Oxalic will not harm the metal anyway. Some really bad tanks can take a couple of days of soaking. When poured out and tank is rinsed with water there's a light haze on the metal like a coating that stays in place, doesn't foul the fuel. It shouldn't harm the tank's paint at all [but you can test underneath tank], or tanks's lining, a big plus compared to a caustic acid like sulphuric, phosphoric or muriatic etc.
Again, you can save the leftovers and whenever a rusty something or other needs a dunk there ya go. Still works fine even if it has turned blackish from chemical reactions.
Nice too that you can get small quantities shipped to you cheap at the link. I have another pound on the way as I write.