I repair heavy equipment such as bulldozers etc which tend to get a lot of rust buildup in the fuel tanks. On one Case dozer which had a 1/4 inch thick 70 gal steel wall tank we removed a full five +gallon bucket of rust flakes and then put two gallons of concentrated phosphoric acid[available from chemical dealers] into the tank then sealed the tank ,attached a rope to it , hung it from a hoist and sloshed it back and forth for about 25 minutes We then drained it and flushed it with water Next we blew it out with air When we opened the inspection plate the inside was still pitted but all the rust was gone and replaced with a grey primer like coating I havent had a problem with rust ever since. I have used this tecnique on several machines Be very careful not to get the acid on you IT IS EXTREMELY CORROSIVE Naval jelly uses a very dilute phosphoric acid as a base