It is important to note that the factory makes no allowance for surfacing the head. It's either smooth and flat or you start over with a new head. This is the "correct" repair but rarely done. If it takes more than a couple of thousandths to clean up the surface you really should find an aftermarket head gasket such as Cometic where you can get a custom gasket that is thicker than stock to make up for the material removed from the head.