The candidates are both commercial plastic resins that can be mixed and poured easily. They have similar chemical compositions, I suspect, though they dry slightly differently.
QuikCast, available from TAP, dries white (about the same opacity as milk). Smooth-On, from Douglas and Sturgess, dries mostly clear. For my application the color isn't a big problem since the plastic will have color added to it.
The plastics were tested on three different types of foams to see how they'd fare.
First up, common polyurethane open-celled upholstery foam. This would be convenient since it's cheap and easy to work with. But it has a very rough surface which makes it harder for the plastics to achieve a nice smooth texture.
Smooth-On (S) and QuikCast (Q) on polyurethane. The thinner areas are a single brushing of the plastic. The thicker spots are where I gave it a couple passes as the resin was beginning to gel.
This low-profile shot shows that the rough texture of the foam affects the plastic's surface. Not what I'm looking for.
Second, I tested the plastics with Ethafoam. This is a closed-cell ethane foam. Also cheap, but not quite as nice to work with.
Both plastics, Smooth-On (S) and QuikCast (Q), skinned pretty well. They could be spread shortly after mixing, leave a good surface, and not show brush strokes. The problem was...
The plastics didn't stick to the foam. They couldn't bond with the foam at all, really. As soon as the foam was flexed, the plastic sheet peeled away effortlessly. So Ethafoam isn't at all useful with plastic resins.
So we move on to crosslink foam. This is a denser crosslinked polyurethane (and other polymers) foam.
This is the best case for the plastics. They form a good surface and adhere tightly to the foam. The only downside is that this foam is about four times as pricey as the regular open-cell stuff. But, hey, it works well.
So there ya go. Hope this is of interest to any fursuiters interesting in building plastic things.