Just to confirm what lots of others have said. Watered down emulsion for a couple of coats ( it dries really quick ) or watered down PVA. It shouldn't take any more than a few days for the plaster to dry out and you will see when it's dry as it will lighten unevenly. It's really easy to see. If you do use PVA to seal be absolutely certain the plaster is dry as the PVA will create a barrier on wet plaster and prevent it from drying.