Rust can be dissolved with strong acids but they also attack the underlying clean metal. Their use depends on how bad the rust is and how much metal can afford to be wasted. The popular, commercial rust removers use milder acids (citric, phosphoric, etc.) and attack rust faster than they do the metal. But they are slow and the job is incomplete.
I prefer sand-blasting.. does a nice clean fast job.. gets into the tiny pits and removes every last bit.
To do a thorough job any tank, including one that's part of the frame, has to have a couple of carefully planned access holes cut into it.. or be opened up at a seam.
Then whatever methods are used... acid treatment, blasting, protective coatings .. can be done more easily and the work can be checked. Afterwards the tank is welded, brazed or soldered back together.