Here is some in progress pictures of the painting of the groundwork.
The groundwork is painted with Humbrol using a no 1 and no 3 brush. The mud will get a more wet look adding some satin varnish to some places.
I think mixing in all those debris make the groundwork more "busy" and interesting.
The groundwork is made of Sculpamold, I think this is a mixture of celluclay and plaster.