I had my wife's grandmothers cast. They had caked on layers of carbon and it was flaking off.
I got a 5 gallon bucket filled 4/5's with water and put in a can of Lye I got from Ace Hardware. I put in the skillets one at a time. It ate off all the crap. Wearing gloves and eye protection through all my encounters with the Lye water. The Lye will eat up most of the carbon and get you back to the metal. Then you can reseason and use like they were new.
You can use the Lye solution several times. Just make sure you add the Lye to water and not water to Lye.
You can pm me if you want. I have a stubborn skillet in lye water for over a month now....
UDS - Midi Me - Weber Performer - 22.5 Weber OTG - Weber Genesis - GOSM
White Lightning Thermopen