Allow me to share a never fail technique to save cast iron. I often buy skillets in antique shops, some are pretty nasty. I use that for bargaining. Anyway, The Pan Man suggests this method, and he's a noted authority on Griswold and Wagner c.i.
This involves nasty stuff, so get rubber gloves and goggles. Find oven cleaner with lye, I only see it at hardware stores. Scrub your pan with soap and water, and dry it. Now slather it real good, all surfaces and the handle, with the oven cleaner. It stinks, and will make your eyes water, so do it outside if possible. Place it in a garbage bag, and tie it up tight. Now let it set for 3 to 5 days, depending on how cruddy. Meanwhile, get a brass brush used for cleaning whitewall car tires. When its time to take out the pan, (gloves and goggles again), pre heat the oven to 275, wash the pan in hot, soapy water and scrub it with the brush. It should like like the day it was cast, nice gray iron with no blotches. Dry it quickly and thoroughly with paper towels, then place it in the oven, where it will dry completely in 15 minutes. Now you can season it, I use Crisco. Wipe it on all surfaces, place it on foil, right side up in the oven. After 30 minutes wipe out the excess and put it back in the oven for 30 minutes. Shut off the oven and let it cool, you should be good to go.
The first time I did this was on a 70 year old Griswold, looked like it had been in a swamp for 69 years. Worked so good I re-did some other pans just for fun. I think it works so well because the chemical get into the pores and grain of the iron, something no brush can ever do.