Whelen Nut. Just recently received an older 20 ga. Topper shotgun barrel with a clean bore but pretty rusted exterior. Blueing was thin but intact in most spots. Made a little trough with tin foil and sprayed some Walmart SuperTech brand spray to free rusted fastners ETC. This spray completely dissolved all the rust on that barrel. Kinda smelled like PB Blaster.
I am now using it on the rusted bore of a rifle barrel. Things look promising so far.
