Upon reading the initial post my concern was brush painting vs. spray painting a uniform thickness of paint into the spark plug hole depressions. I have never spray painted engine heads but have dealt with the care needed in spray painting convex and concave surfaces. Regardless, it sounds like you "sprayers" have had no problems in doing the spark plug hole task. I'm old fashioned and my personal preference for this kind of task remains with brush painting since it reduces or eliminates the time needed for masking work and gives me better control over paint thickness.