I personally don't like to seal plaster before painting. I use thinned down water based craft paint. It seeps into the plaster and give it a nice, realistic look. If you seal it first, the paint will just sit on top of the primer and look more like painted plastic.
Go to <Railroad Line Forums\Forums\The Craftsman's Corner\Seal or no seal test> for the current discussion on this subject. I always seal with a light coat of Rustoleum gray in a rattle can. This becomes the morter lines for a brick wall.
Wayne
Having built a few of Randy's Downtown Deco kits, I follow his advice and lightly overspray the casting with either a clear finish or a white paint from a rattle can. You don't want to completely coat the castings, just a light overspray. The water based paints will still soak in, but you will have a bit of working time.
Drew