Kneadatite (more commonly known as "the green stuff") is used a lot in sculpting gaming miniatures. I have sculpted several out of this stuff. You can get some at just about any gaming shop that carries Games Workshop miniatures. This is a smaller amt. than what you can generally buy online, but if you don't need alot this may help.
The only thing you want to know is that when you use it it wants to stick to absolutely everything. More often than not it will stick to the tool you are using to put it on with rather than the part you are trying to put it on.
You can dip your tools in denatured alcohol, or use the "spit method" (Yeah, I lick my sculpting tools...so what?!?
) to alleviate this momentarily. But you have to keep doing it over and over.
You may want to try Bondo. If you've never used it before, its a quick mix, fairly easy to sand, and inexpensive (@ Wal-Mart) filler. Plus pick up a tube of Bondo glazing putty. This stuff will fill in any small holes or rough areas and sands like an absolute dream.
Hope this helps some.