I routinely inspect my molds for rust, and even though they're stored in a strictly climate controlled room now and then some surface rust pops up. Mainly because I never ever apply oil/grease as a preservative. I just use either 0000 oil-free steel wool or the gray ultrafine 3M pads to whisk it off, paying close attention to making sure the vent lines are clear too which I do with a fine point stylus. Now and then though old iron molds fall into my lap that sport heavy rust and as long as there's no pitting in the cavities I've usually resurrected them using techniques much as HawkI delineated. If the cavity is pitted, it goes in the junk drawer along with the rest that are saved for use as the basis for re-cutting to custom cavities, which never happens.
The best solution though is to not let it rust in the first place. Or better yet go with aluminum molds.
