Kevlarr is dead on here, the file is a bad idea for working on an aluminum head even if you know how to run one your chances of getting it as good as using the glass and sandpaper is slim.
If you dont have a piece of glass it is sold very cheap at home depot, If you dont have access to a milling machine this is the best way to surface the head and get a god level gasket surface, just be sure to always apply the pressure straight down centered on the head and move it in a circular motion, if you push down on one corner more than the rest even this method can work the head down unevenly just take your time and apply even pressure, and you will be in good shape.
Good Luck...
