Originally Posted by mapbike
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.
A good way to get uniform pressure: first make sure your glass surface is both DEAD FLAT and also DEAD LEVEL if you can. Then put a plug part way into the head, and put a five pound dumbbell weight over the plug. Then don't press down at all. Just move the head around on the sandpaper very carefully as described above, not by pushing on the weight or the plug - just handling the sides of the head. I believe it should provide a uniform pressure evenly across the mating surface and you won't even make your wrists sore. (I have bad wrists, that's why I come up with stuff like this.)