Most just use #0000 steel wool to remove the rust. Once the rust is removed and there is bare metal showing, rub with bacon fat, olive oil or canola oil. Let it soak into the metal for a day then wipe and fire it up. The heat somewhat cures the oil into the metal almost like seasoning a cast iron pan. The best prevention from further rusting is to not let ash sit in the kettle during the winter or when it rains. Water mixed with ash is very caustic and will eat away at bare metal.