Naval Jelly, available at home improvement stores, is great for removing rust. Scrape off the loose rust, brush on the NJ, wipe or rinse of the next day. The rust (oxidized iron) will have been reduced (opposite of oxidized) by the phosphoric acid in the NJ. Wear personal protective gear when handling NJ. Gloves, eye protection, etc.

Put 3 foot chain that go into tank opening after take off shake in every direction take chain out tank rinse with gas. Then add some two cycle gas to tank rinse tank again this colt tank with oil if cap rusted may want get new one they hard to clean. More weighted chain you get in tank faster and better rust come off. O'Relys has liquid rust remover but need be in contact with rust for 12 hour. rocks work like one over in fish tank section of store, tumble tank lone as can. I have old Gilson tractor and take 2 small load straps strap to rear wheel drive it around put hold bag rocks in tank more rocks better that shine tank on inside.