A rust killer, or converter, chemically converts the rust from iron oxide to iron tannate, which does not corrode the metal of a vehicle. A good rust converter for cars is Corroseal, which can be purchased at hardware stores by the gallon.
Rust killer is a good option for eliminating a small area of rust on a car, and removing the rust can be done at home. Rust killer not only removes rust, but it also contains a primer coating to prevent future rust from developing in the same area. Corroseal is easy to use and compatible with most cars.