Originally Posted by jennyema
Lordy ! It's simple
Hot soapy water ( dish soap/detergent ) scrubby/scrungie
I'm a bit skeptical about hot soapy water harming a cast iron skillet other than temporarily, despite what some have said. When mine has too much crud I get out a plastic scrubber and a bunch of dish washing soap and scrub away best I can, then run it through the dish washer.
The next few times I use it I oil it to keep the food from sticking, then clean it with a scrubber and plain water. After a few oil, cook, wash with water cycles it's as good as new. Good as old. Whatever.
The problem the OP has is rust. I'd tackle that with a wire brush and steel wool, then run it through my suggested method above.
And yes, whoever said it, of course if you heat it until it's red hot then throw it in ice water it will probably warp or shatter. You could drive a caterpillar over it and destroy it. I meant you wouldn't destroy it doing reasonable things.
It's amazing to consider that cast iron was the original non-stick cooking surface. The Iron Age began about 1,000 BC (possibly much earlier) and lasted trough about 700 AD, and cast iron cookware might have been in use around the time of Jesus.
And it's still such a good cooking utensil it is worth having in our modern day kitchens!
(No comment on the ammonia method since I've never tried that.)