Make a solution of two parts white vinegar and one part water. As much as you can, clean any old grease from the cast iron pan you wish to clean. Soak the item overnight in the solution. The vinegar will dissolves the rust from the rusty pan. Heavy rust may take extra soakings. Do not overdo the process since you are dissolving a thin layer of the metal. Do not let Vinegar Magic make your pan disappear!