Diff Housing cleaning

That and naval jelly and lots of elbow grease. That type of brush fits on the chimney sweep stuff at your local harbor freight, and I'm wanting to say they have some steel brushes similar to that. A large drill makes the magic happen a lot faster, too. I like to degrease with oven cleaner (caustic soda) and hot water and then attack the rust with phosphoric or hydrofluoric acid based cleaners. Gloves, eye protection, and a mask are all mandatory. Rinse the water out with WD-40 and get a coat of oil or preferably cosmoline based protectant on everything. WD-40 chain wax is pricey but a great cosmoline rust preventative.