use muratic acid you can get it at any swimming pool supply store, eats rust fast but it stinks, use plenty of caution and have lots of water around to flush it away if you spill, use a plastic wash tub for the outside or pour acid in tank to clean inside leave in for 20 minutes - 1 hour depending on strength of solution and severity of rust flush with water and baking soda, been using this in tractor gas tanks for 35 years.
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