KUU diamond file.
These files come in various grits, for most people a course (blue) or a Extra course (black) diamond file is best.
The blue will take longer to use and removes less material, but will last longer if used more frequently on skis that are already well tuned.
The black will remove burrs quicker and will get slightly duller skis sharper quicker, but the file will not last as long as the blue one.
Diamond files work better and last longer if used wet, so have a container of water handy when using a diamond file and dip the file into the water often.