Ol man wrote:
The heads I was doing were flesh colored plastic with medium grey paint for the helmet. Even while plunged in the clay you could start to see the 'shadow' of the painted part, and stop with a few mm left. That said, it did take several removals from the cap to check the fit. I just can't seem to hold the hollowed out head with my fingers without the dremel flinging it out of the way. If I grip tighter, the helmet squishes closed too much. It is interesting to hear all the different methods out there though. I haven't tried a sanding wheel to smooth out the inside, figured the grit would pack up too fast with that soft plastic. Might have to try and see.
How many fingers are you holding the helmet with? If it's just your thumb and index finger I can see this being a problem, but I get a full grip on it with all five fingers at once. But, if you've got a method that works for ya, stick to that.