Almost immediately we could see that the rust was deeper than we expected. After seeing the extent of the panel we needed to prepare, we decided to use an electric grinder with a pad attached. With the grinder, the paint easily flew off the roof. A word to the inexperienced: You can expect more harm than good from using these grinders. Take it from us. If you apply too much pressure or concentrate on a single area for a little too long, you'll end up with flat spots.