Ah, easy then. Just give it a good scrubbing with some crumpled up aluminum foil and a mild acid such as lemon juice or white vinegar. Or, if the rust is light, you may be able to skip the mild acid - I've had pretty good luck using the crumpled aluminum foil with light machine oil, which has the benefit of leaving a very thin coating of oil on the surface when I'm done, to inhibit future rusting.