what actually works to remove rust from cast iron skillet?

steel wool and some elbow grease, honestly. for bad rust a soak in half vinegar half water helps, but check it every 15 minutes or so and don't go past an hour, it'll start pitting the iron. then wash, dry it on the hob, rub on a very thin coat of oil and bake it upside down in a hot oven for an hour. wipe off as much oil as you can before it goes in or it turns sticky.