You can remove rust from cast iron skillets using the end of a potato and coarse salt. This combination removes rust easily, according to Apartment Therapy.
The end of the potato needs to be large enough that you can grip it firmly in your palm. It serves as a scrubber in place of a traditional sponge. Set the cast iron skillet into the sink and add about 2 tablespoons of coarse salt, sprinkling it evenly around the rust spots. Using the potato, scrub the rust spots thoroughly. The abrasiveness of the salt, coupled with the moisture of the potato, removes the rust. Rinse and repeat as needed until the rust stains disappear.