Use the Clay node, instead of Diffuse, with high roughness or whatever it's called on the clay node. No specular at all. Need some roughness to the surface - you'll want some bump. Not sure which kind of plaster this is. Molded? Need bubbles in it?

I am sort of busy otherwise I'd help more.

 

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)