Okay, so apparently PVA mold release doesn't WORK for HYDROSTONE. Sure, it worked for Ultracal (I think I used PVA)... but I had to beat my three-part mold into little bits with a hammer to get my arm out.
Okay, not quite. Lots of little bits and a few big ones.
And the arm isn't out yet.
I had about a 30% "stick" rate, it seems...
!@@#$%@##@ plaster.
I'm going to either try again with (a) a different mold release, (b) a different plaster, or (c) screw the hard stuff entirely and use a firm polyurethane rubber mold.
Dammit.