Oven Cleaner Method

1. Spray a heavy layer of Easy Off oven cleaner on the cast iron. This can be used to clean off heavy carbon build up, but will not remove rust.
2. Place your cast iron in a trash bag and sit it outside in the sunlight.  This will work in the cold, but works best on warm days.
3. After 24 to 48 hours, you should be able to wipe it off or use a SOS pad to scrub the carbon off. If any is left, simply repeat the steps until clean.
4. Once clean start the seasoning process.

*** NOTE: Make sure you use rubber gloves so that the oven cleaner does not get on your skin.

Credit: Cast Iron Community