We've bought cast iron skillets at garage sales, sometimes in awful condition. We stick 'em in the self-cleaning oven and burn off all the old seasoning. They emerge clean, but probably rusty.(posted 8873 days ago)I prefer not to season them in the oven, but to simply put them on the stove burner and heat them until very hot--then wipe them with a paper towel, wet with olive oil. Any cooking oil or fat will work, but olive oil seems to work best. As the excess oil begins to break up and coat the skillet unevenly, wipe again with the paper towel, adding more oil as needed. Several coatings using this procedure will produce a good cooking surface. The surface should be black, smooth, and have a low lustre.
We never wash cast iron with soap or detergent, just with hot water. A "Tuffy" pad will remove any stubborn food residue. After nearly every use, we set the cleaned skillets on the stove burner, heat them, and use paper towel to wipe on a light coating of olive oil.
Cooking anything that holds oil in contact with the hot skillet also helps the surface. Pancakes or crepes, cornbread, etc. are excellent skillet seasoners. Of course, you'll be forced to eat the results, but maybe the sacrifice will be bearable.