To remove rust on the outside of a tank, I recommend a light sandblast, but not with enough pressure to deform the metal. Metal can also deform from heat generated by the friction of too much sand hitting it too hard for too long. Then repaint.
Here is aa sandblaster for $19.95 – and get the blasting sand at Menards. https://www.harborfreight.com/21-oz-hopper-gravity-feed-spot-blaster-gun-95793.html?_br_psugg_q=sandblaster
If the rust is on the inside of the tank, somewhere on this site there is an electrolysis procedure using an automotive battery charger that works pretty slick. After rust is removed inside the tank, there are several tank coating products made to keep rust from coming back. Most of these products are marketed for motorcycle tanks.
Dave
