Bob - my advice might be unusual, but here goes:

I don't prime the plaster at all. I airbrush the paint on. I use acrylics, so they dry quickly. Get a section of broken plaster or a plaster concrete block and experiment. Once the paint is dry, I like to use diluted oils to weather and tint the parts of a plaster building - like staining on concrete. Apply it like you would a wash on a tank model - all the way from the small "pin" washes to the wide-brush washes depending on the surface area. If you experiment on a little section or concrete block, you'll get a handle on how much and where you want to apply the oils for staining. Since the plaster is pourous, it will absorb the wash quickly, allowing you to layer colors.

When that's dry, I then will go back with pastel chalks for surface weathering - a tiny bit goes a really long way if you did all the above. The little wall along side the Kubelwagen above was done this way - with one color, four washes of black and brown oils mixed in different ratios, and two colors of pastel chalks. It only took 5 minutes for the whole sequence.

Gunnie

Hi, may I know how did u make the dried leaf in ur diorama? they look realistic!!