Here is how I cleaned the mating surface on my head. I was unsure of how to do this as it is alloy obviously and I was afraid of damaging the surface so I was about to send the head for skimming.Then an old boy who visits the garage from time to time told me to put the head on a sheet of glass coated in fine vavle grinding paste and oil. It worked a treat the surface looked like new.
http://www.youtube.com/watch?v=ru9vBNk0_88
http://www.youtube.com/watch?v=ru9vBNk0_88