I´ve been restoring bikes for nearly 20 years now and after numerous trials and errors I´ve found that Muriatic Acid works best. Muriatic acid is often use to clean aluminum siding before repainted. That how I discovered it. Put a few nuts and bolts in the tank and maybe a third full of muriatic acid, shake it a bit and drain it out. Muriatic Acid is available at just about any store that sells house pain. Wear Gloves or something because it is an acid and can cause acid burns on your skin.