I use one of two methods. First to give it a bath in "iron Out", sold at the big box hardware store to remove iron rust stains. "Zud" will also work. Use a plastic bucket, put the item and some of the cleaner in and add hot water. Do it outside, don't let animals or children or such near it, stuff is corrosive. Let it wait a few days, then take a 2 ounce tack hammer and after washing well, knock the loose rust off. You can manually remove most of the rust.
2nd method, called the shortcut, heat in a campfire and then douse it. Can be hazardous but the quick contracting a lot of times will let the rust flake off. Then use the tack hammer to finish it.
Both methods of course have to be done outside.