The best way to restore cast iron pots and pans is to put them on a bed of coals and slowly heat them to a dull red heat. Do this carefully and evenly to avoid cracking. I've never had one crack, but if the heat were not even it might be possible.
This will also remove all built-up crud. After this process they need to be seasoned. You'll essentially have a pan as it came from the casting process. If you have access to a heat-treating oven (furnace) that is large enough and will get to 1200F, that would be the best way.