To paint new plaster, the surface should be sealed using a solution of a PVA adhesive especially if you plan on putting oil based paint on the surface. Some companies produce their own plaster sealer which you can purchase. When the surface is sealed and dry, you can now start painting. Do not paint over wet sealer. If it is water based paint, then mix some of the emulsion paint with water at the ratio of 1:4 and then paint it on the plaster to act as a sealer.