I've used the electrolysis method before. I found it pretty difficult to suspend an anoide of sufficient mass inside the tank. If I had to do it on a tank that I care about the paint on, i would chemical strip it with acid or another method. If you don't care about the paint, you can get a big rubbermaid tub, fill it with water and sodium carbonate, and drop some steel rods into the tank as your anodes. Much much easier, but will separate the paint.
Be forewarned, this method works pretty well, but its going to have a hard time knocking loose any heavy nodules of rust, its more aimed at getting rid of relatively minor surface oxidation.
