Cleaning rust from cast iron using a potato and coarse salt is a simple yet effective method. First, cut a potato in half and dip the cut side in coarse salt. Rub the salted potato vigorously over the rusty areas of the cast iron skillet or pan. The oxalic acid in the potato, combined with the abrasiveness of the salt, helps to lift and remove the rust without damaging the seasoning of the cast iron. After scrubbing, rinse the pan with water and dry it thoroughly. This method is natural, inexpen