I'll pretty much second what Trigger said. I've used several resin heads and I have always done the following with no problem with the final product:
1. Sand the head with a very fine sandpaper like 360 or more for grit
2. Wash the head with soap and water to remove any mold release (yes, casters use mold release just like the factories)
3. Prime the head. If you ever want paint to properly adhere to anything, primer is your best friend.
4. Seal the figure or have some sort of varnish finish mixed in with your paints. Acrylics need sealer if they are going to stand the test of time.
You are using acrylics, right?