With the HF poly carbide disc, be sure to wear a dust mask and dust will get everywhere. So do it outside. Try to keep the disc fairly parallel to the surface. It will last longer that way. Once you try it you will never go back to a flap disc for flat metal surfaces. Keep the disc moving so you don't build up heat. You can go back over heavily pitted area and remove a lot of the rust. Then go back over those pitted area with media blasting.
The disc will save you a lot of time.
