Yesterday this was a nice clean casting of my original sculpture (above left). Now after taking an electric saw, hand saw, power drill, and various Dremel bits to it, it's an official wreck (above right).
No panic here because after cutting off the back of the head, poking out the eye sockets, cutting out the jaw cavity, roughing up the interior, and smoothing the edges all around, I'm usually left with some patching. Sometimes the hand slips or the plastic gives way unexpectedly. Nothing a little paper clay or Magic-Sculpt can't fix.
The first step is always a little nasty.