I've use the POR 15 fuel tank cleaner and sealer, seemed to do a good job. The old "system" I've also had success with is to shatter a side window out of a car, put the pieces in the tank and shake/roll/agitate the tank with all those super sharp shards of glass in it....Kind of tough to get all the glass out of the tank but it sure does do a heck of a good job cleaning the rust and other crud out of an old tank!