Oh I got some really nasty ones once.
They look new now though.
I used lemon and salt. I started with table dalt and moved on to rock salt after 3 pans. Just cut a lemon in half (gonna need a couple) and rub it through the salt.
My grandaughters love my cast iron. They use the ones I bought when I moved away from home at 18. My oldest daughter is a clean freak way worse than I used to be and almost threw a skillet out. My grandkids said "NO! grandma can clean it!" They all know the value of cast iron! YEAH!
I love when people get rid of cast iron. hehehe. I usually get it for free.
If it's really bad you can also use sand. YES sand. wet it just a bit and use a soft brush and scrub.
Just make sure to season it. I use coffee filters between mine so the stay nice.
Try the lemon and salt!