I don't know how you would do this without filling in the scratch grove with the same metal material the back is made of (or sanding the surrounding surface down to the same depth). It would still be noticable. But why bother? No one can see the scratch when you are using it, since it is in your hand. This is a functional device afterall, not a piece of art in my opinion. I would spend the time using it rather than trying to keep it looking new.