Electrolysis ! it does work.
I remove all the rust out of all my tanks, and all rusty parts using the electrolysis method.
Type in electrolysis on u-tube . Their is a guy working on a old steel 5 gallon boat tank on their. Take a look.
This is a ( on site) method, so if your tanks have baffles, you will have a problem unless you can gain access to the other side.
I did a old Ford 8N tractor fuel tank going through the fill opening with a electrode, and going through the drain plug hole. Took two complete days but, it was clean and shining like a new nickel.
All you need is water , WASHING soda , battery charger 10amp, and a electrode. I use old rebar or old long gutter nails. (No aluminum !!)