I've restored a number of old skillets I find at flea markets and such. Get the Easy Off oven cleaner with lye, I only found it at a hardware store. Slather it on heavy, all surfaces, wearing goggles and rubber gloves, do it outside. Then stick it in a garbage bag, and tie it shut. let it soak for 3 or 4 days. Scrub it clean with soap and water with a brass brush used for whitewall tires. Put the pan in a hot oven right away, to dry it completely, or it will flash rust right away. The pan should be virgin gray iron, and you can season it. I'll pass on a foolproof initial seasoning technique if you'd like.