Note that whatever you try will cause any rusty spots to get very thin or even leak. I did a motorcycle tank about 5 years ago with POR-15 and it still is holding up, but I made sure I got the entire thing well coated. I must have tumbled that thing over and over for at least an hour, it was tedious. I used phosphoric acid to etch it first. Electrolysis may get rid of most of the rust but it may not provide enough tooth to adhere the coating, at which point the tank is ruined. Make sure to etch the tank with whatever the coating says to use, and follow the instructions to the letter.
I wouldn't do it again unless I had a tank that I couldn't get a reproduction for. The bike tank was one of those.
