Cro 's right. You want to use diamond stone to polish. A file will take off too much metal and you'll end up needing base grinds and your sidewalls planed to keep it flat with the edge
I personally use different bevels on my boards so I use an SKS tool with the file to set the angle and then polish and sharpen with a set of diamond stones. The tool just helps to stay consistent. I use it so much that it's more than paid for itself.