Bicycle chain. Ever tried to get that last nut/bolt/bb/rock out? Bike chain is the way to go, then phosphoric acid, it will put a skin-coat of ferric phosphate on the steel protecting it from rust. Or...... You can mix up some electrolyte (water plus washing soda) fill the tank, suspend a cast iron doohickey in the tank NOT TOUCHING ANY PART OF IT and hook a battery charger to it. Neg lead on the tank, pos on the iron. The electricity passes from the tank to the electrolyte, picks up the rust and sticks it to the iron. Use iron or mild steel, no stainless. Graphite works the best but I doubt you have a big chunk of that handy. Clean your annode daily and change the fluid every two days6-10 amps is good current. . Produces hydrogen gas in small quantities so put a fan on it.