From Lodge
While maintaining the seasoning should keep your Cast Iron and Carbon Steel in good condition, at some point you may need to re-season your cookware. If food sticks to the surface, or you notice a dull, gray color, repeat the seasoning process:
* Wash the cookware with hot, soapy water and a stiff brush. (It is okay to use soap this time because you are preparing to re-season the cookware).
* Rinse and dry completely.
* Apply a very thin, even coating of MELTED solid vegetable shortening (or cooking oil of your choice) to the cookware inside and out. Too much oil will result in a sticky finish.
* Place aluminum foil on the bottom rack of the oven (not directly on bottom) to catch any drips.
* Set oven temperature to 350 – 400 degrees F.
* Place cookware upside down on the top rack of the oven to prevent pooling.
* Bake the cookware for at least one hour. After the hour, turn the oven off and let the cookware cool in the oven.
* Store the cookware uncovered, in a dry place when cooled.
* Repeat as necessary.