This is rather extreme and I would only do it if every thing else fails.
If you have a self cleaning oven, put it in upside down and let it run through it's cleaning cycle. Leave it in the oven until it cools. Now after doing that you will have to re season it. Maybe even two times.
I've done that with some old cast iron skillets I got at the thrift shop. worked beautifully.
Of course I had to put a fan in the kitchen window to clear the smoke out from all the crud burning off.
