Re: Old Mossy's paint log - Update 13/2: Skrox and a water b
Do you mean real clay? I was considering using miliput for the next batch. It'd give more control over the contours of the shore than the foam, but would probably cost more per square foot of ground to use.
To answer your question, I'm using Extruded Polystyrene Foam (not Expanded polystyrene; that's the stuff made of small 'bubbles' of foam). It's pretty durable and has good texture. It's really good for producing 3D objects (eg. boulders, cliffs, ruins, pillars, etc.), rather than more 2D ones (like the ground). Not that it isn't fine for 2D, but I just think there's probably better tools for the job.
You don't need to seal it, but it does definitely give a tougher outer shell if you do. I usually just mix 50/50 paint and PVA glue, and paint the base coat to seal my stuff.
Caution: If you are going to spray paint the foam, you MUST seal it before spraying, otherwise the aerosol will melt the foam (a lot!). Again, you can seal it just by giving a good coverage of slightly watered-down PVA glue (if you mix paint in with the glue, you can see where you've missed).