Hi, Nelson. When you sand something--wood, metal, whatever--you scratch it. If you want to polish the item, you go to finer and finer sandpaper which takes out the larger scratches with smaller ones. Then, depending on the material, you can polish, which puts in microscopic scratches.
Without seeing the piece and the scratch, my guess would be that you would want to start around 320 grit wet/dry paper, then 400, then 600 wet. Then, if you want to, you can go to a polishing wheel and jewelers rouge.
Hope that this helps but let me know if you have further questions. Henry in MI