The problem is?
(a) It's going into the plaster
(b) It's not going into the plaster?
If you want it to go in the plaster, diluted the paint/stain with water. Experiment a bit and you'll find the correct ratio water-paint. For me it's about 4-1 (4 part water for 1 part paint)
If you don't want it to go in the plaster, than it's the opposite. Make sure the paint/stain is not dilluted. That way it will cover the plaster but will not soak in.