Hello,
yes I use that stuff every day - it is called climacell (see www.climacell.de for further information). Although I thought it will give a rather rough surface it is really smooth. And very easy to sand down to get a really smooth surface - nearly like a tissue-pulp-finsish.
When dry it is quite "soft", so you better add sawdust etc. for stability.  I like it because it is easy to handle and it is mixed with borax to avoid mildew and parasites. It is even fire-resistant. A goooooood stuff. I buy it for 1 Euro per kg.


hth

Christina