x2 what Johno02 said...
no need to use chemicals you can clean/restore even the worst condition cast iron with heat and oil. A good hot fire will turn the rust to dust and burn off anything on it, bury it right in the coals when you heat it. The rust should just wipe off with a green kitchen scrubber after it cools. When you reseason with oil and heat a second time, it will look and work good as new. I have heard of people passing the same pan down for generations and they still look/work like new. I have also heard of people finding old pans that have been sitting in a shed for decades and are covered in rust and making them like new again.
I think the only way you can ruin it is by cracking it.
Once restored, never wash it, just wipe it off with a green kitchen scrubber. For really stubborn messes if you really must, use a small amount of soap and scrubber and after rinsing put it back on the burner to dry however you may have to reseason it more frequently.