I've always used those metallic paints that you can add a solution to to create rust. You paint it on, let it dry for 24 hours then hit it with the solution and it oxidizes pretty quickly. Added plus is since you're applying a paint, you can add rust to just specific spots.
Another method I used was undercoating the whole part in a rust colored paint, then before spraying it with the main color I add some water to where I want the rust to be, then sprinkle table salt on the area. Once the salt is dry, I apply the main color, usually by rattle can or airbrush. Once that's dry I take a stiff brush and scrape off the salt. It leaves patches that look like they've corroded through the top surface.
