Plaster walls generally require at least 30 days to dry fully, and under cool or moist circumstances, full curing can take months. Painting over new plaster too soon traps the moisture from the plaster under the paint, leading to mould growth. However, with the proper preparation and choice of paint type, you can reduce the time you need to wait before painting over that new plaster.
Cover the floor and any surrounding areas with protective plastic or dust sheets to prevent dripping or spilling the primer and paint.
Prime the plaster surface with masonry primer or with a mixture of four parts emulsion paint with one part water. Paint on a thin layer of the appropriate primer; you will see and hear the plaster absorb some of the moisture from the sealer. This will prevent the paint from completely covering the plaster.
Paint the plaster with microporous paint, available from painting speciality stores. Coat the wall in one thin layer of paint, allow it to dry, and repeat with a second coat. These paints do not completely seal the surface, but allow the plaster to continue to breath so the moisture is not trapped.
In general, masonry primer can be applied when the plaster has dried for seven days at 25 degrees Celsius in 50 per cent humidity. Lower temperatures or increased humidity will extend the required waiting period. If your plaster is soft or powdery, mix 1 pint of vinegar with 1 gallon of water and spread it across the plaster surface. This will harden the plaster. Rinse the area with water once it is hard and continue to paint. Microporous paints often require you to paint over them with regular paint once the plaster has completely dried, to ensure a full seal on the wall. Follow specific product instructions or consult a professional at a paint supplier for details.