two ideas

If you're not opposed to repainting the tank, have it derusted by a Redi-Strip franchise, as advertised in Hemmings. Will require soldering the seams and repaint. Most electroplating shops have the a sodium cyanide vat that is same process. Muriatic acid works OK too for home use if you have enough to submerge the whole tank. Alternately, this is what worked well for me. Use a powered, vertical turn table. An old cement mixer is perfect. Lash the tank to the turn table. Partially fill tank with ceramic media and correct PH solution. Same as used in industrial vibro finishing. The correct solution will help dissolve rust trapped in pits. After, fill tank with alcohol to check for leaks. Is more viscous then gas.