I've been doing anodizing for years. Heads can be hit and miss, it depends on the grade of aluminum. If machined from 6061 or 7075, you can get good results. If its cast from high silicon aluminum (and many are) all you get is a nice, dark grey finish that won't take the dye. The dyes are special, very fine grain products with the correct PH.

Any part that isn't meant to be treated, can be masked off with a soft epoxy mix that doesn't fully harden. Depends on the brand, but mixing only half the hardener usually works. That peels off easily when the anodizing is finished.