Any polish probably won't get out the scratch, but pics would definitely help out. What type of knife? If we knew more, we can give you more help. For now, I would say you will need some grit sandpaper, the same grit as the knife has on it now, and basically reapply the finish. Very easy to do, but will need the right grit paper, along with a hard sanding block to keep all the nice straight grind lines.