If it's a really bad scratch, you can start by using some wet dry sandpaper. First use a 320 or 400 grit and gradually work to a fine grit like 1200. Then use your Dremel tool with some really fine polishing rogue made for metal polishing to buff out the fine scratches left behind by the sandpaper. It's best to use a different buffing wheel for each grade so that you don't introduce coarse grits on the final buff and ruin the mirror finish.