For very mild cleaning of motorcycle tanks, I've used vinegar, just pour a gallon in, fill it up the rest of the way with water, and let it set for a day. For tanks with a little more rust, go to a farm supply store and buy "Milkstone remover" or "The Must for Rust" from a Home improvement store. After using these products, rinse really well with water and then coat the inside with a light oil to prevent it from flash rusting, and then keep the tank full all the time.
As far as coatings go for fuel tanks, I've tried a few, and the only one I will ever use again is the POR-15 kit. This really works good, but you must use the complete tank restore kit (has 3 or 4 different components.) It is absoutely a permanent fix when done correctly, and the tank will never rust again.
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