From the wording of the question from 420SIX, I am assuming the metal part in question is a piece that will be painted. If that is the case, then a product with phosphoric acid will give primer and paint better adhesion and offer long term durability for the finish. If the part is plated or natural finish, molasses or oxalic acid are two options for rust removal. I used oxalic acid on a Barracuda rear bumper. As stated by others, I built a vat with lumber and a poly liner, or paint "drop cloth". After rust removal, I treated the bare metal with Boeshield T9 Rust and Corrosion Protection.

David