I am working on a non-modding project which is about 2 foot tall. I typically like using Sculpey but based on the size it wasn't practical. Enter Giotto Plastiroc. It's a paper clay that has its uses.
Plastiroc has the consistency of stiff play doh (about like plasticine and wetter than sculpey) but shears like it has paper pulp in it. Running it through my pasta machine took some trial and error but at the largest setting it went through ok after kneading it flat.
Working it was was easy. As it's water based, you can cut it and clean up with water. I used a set of wire loop sculpting tools. It smooths very cleanly with a water spray bottle and slight finger pressure.
The downside to it is that is doesn't have very much structural strength. An 1/8" sheet breaks like fired ceramic clay (about half the force, or less, required to break Sculpey clay). It does shrink and I noticed cracking for long thin pieces but it fills quite easily and I had no problem putting multiple layers down once I wet the fill piece with water.
The upsides are that it sands almost like talc. I was loving it when I started cleaning up the sculpture. It takes texture well. Using a set of plastic texture plates I was able to lay down a fairly consistent texture.
It also takes color like paper does. When I get to painting, I'm going to experiment with an ink spray to keep as much of the texture as possible. Finally, it air dries in about 24 hours depending on the thickness. No baking required.
In summary, it's not a replacement for Sculpey but for large flat surfaces, like the side of a case, it is a decent alternative that doesn't require baking and is a little more forgiving than Bondo.
Oh yeah, the project is a gift box for a friend inspired by a Goby fish. The head lifts off to put the gift inside.