I've used distilled white vinegar and hot water followed by hot water and coke/mountain dew. The majority of soft drinks contain phosphoric acid, which you can buy at most automotive paint suppliers to remove surface rust of body panels prior to priming. Where as the straight phosphoric acid might mess up the paint on a tank, vinegar or soft drinks won't.
I want to try electrolytic derusting using washing soda.
https://www.stovebolt.com/techtips/rust/electrolytic_derusting.htm
On an episode of "Cafe Racer" I've watched as they poured the solution into the gas tank, hook up the battery charger and leave it for a couple of days. The results looked promising.
Steven