We find that most minor rust can be removed with hot water and a scouring pad. You can add a little salt for extra abrasion, if needed. For major, all-over rust on a cast iron skillet, you can soak the pan in a solution of 50% lukewarm water and 50% white vinegar. Check the pan every 20 minutes or so, and remove from the solution once the rust can be scrubbed off. (Don't soak more than 8 hours, or the skillet could be damaged.) Rinse, dry, and re-season the frying pan following the directions above.