For rusty tanks, pre-gen ones are especially thin. So I'm scared of anything that has mechanical scrubbing & impact or removes more than just rust (most acids). Only acid I've found that removes only rust and stops at good metal is
phosphoric acid, active ingredient in Naval Jelly. Also leaves midly-protective phosphate coating.
I think even better is electrolysis:
https://www.instructables.com/Electr...-Rust-Removal/
https://www.motorcyclezombies.com/re...rust-gas-tank/
http://www.secondchancegarage.com/pu...cleaning-1.cfm <-- a lot of geezers with pre-war classics use this process
Afterwards, DON'T use any kind of thick sealing stuff, they end up peeling off after while. Just POR-15's tank-treatment as it's chemical conversion for surface rather than coating on top.