Yap, plaster requires special treatment before you proceed. First of all you have to make it stop soaking, you can do that by simpley apply a thinned laquer ( i use Nitrocelluloselaquer, but a thinned acrylic version will do the trick as well ). Do that so many times until it stops to absorb anymore. Then its ready to prime and paint.
If you want to do it the totaly unlaquery way, you can do it with gelatine, the one you use to make caketoppings, and just mix it with water, make it hot ( 50 degrees) and apply several times onto the plaster, agin until it stopps to absorb, hope ii could help!
Lord Flasheart: And always remember - if you want something, take it! Bobby!
Bob Parkhurst: [enters] My lord?
Lord Flasheart: I want something.
Bob Parkhurst: Take it!
[she undoes her shirt - exeunt]