I believe radiators shops used or still use sodium hydroxide (lye) to "boil " out the radiator. The lye removes grease and some crud but hard deposits might need to be physically removed ....Lye will corrode aluminum radiators...You can do this at home, some hardware stores still sell lye for cleaning plumbing pipes. Lye is obviously cause severe burns, you need face and arm protection.Lye will not corrode steel so it can be used in a fuel tank...I have used muriatic acid on old radiators and fuel tanks...Again it nasty stuff ... Some or all fuel tanks are galvanized inside to inhibit rust...The coating breaks down and rust will likely return..