Plastered walls accept paint well. Because plaster is porous and textured, it's simple to create a smooth, even layer of paint of the depth desired, as long as the plaster has dried completely. Because of its drying process, new plaster will leech away any moisture placed onto it. Try to paint the plaster before its six-week drying process completes, and you end up with a thin dry layer of paint that soon flakes away. Sealing new plaster requires filling the pores with a material that can harden the skin, creating a surface suitable for accepting the topcoat paint.
Mix water-based paint with water to dilute it for application as a sealer onto your newly plastered walls. Use PVA adhesive instead as a sealer for oil-based paints. Use a ratio of four parts water based paint to one part water. Use the same mixing ratio to thin the adhesive. Use a mechanical mixer or wooden stirring stick to dilute your mixture thoroughly.
Mask off the wall by applying a strip of masking tape to all adjacent surfaces. Place a dropcloth along the base of the wall as well to protect from spillage.
Test the sealer in an unobtrusive corner of the wall. Apply a small section of the sealer to the plaster with a paintbrush and wait one hour to note the results. Look for absorption speed of the liquid into the plaster. The edges of the area where you placed the plaster will show signs of discolouration if it absorbed the liquid. If the surface right outside the test area remains dry, add more moisture to the mix by increasing the ratio of water. Continue testing with the maximum ratio usable being equal parts paint or adhesive and water.
Pour the newly mixed sealer into a flat paint roller pan.
Cut in the edges of the wall with the sealer, using a paintbrush. Paint a strip along the edges of the walls about 3 to 4 inches thick with the brush, fully covering the plaster. Switch to the paint roller and apply the sealer to the remainder of the wall surface, working vertical rows along the wall length and overlapping each of those rows by about 3 inches to ensure that you leave no gaps in the coverage. Wait 30 minutes for the sealer to begin absorbing into the wall.
Repeat the application process with a second layer to ensure complete coverage. Allow the second layer to dry completely before painting. Drying times will vary according to manufacturer or temperature of the room.
Choose a paint for application onto your wall after sealing, either a water-based emulsion paint, or an oil-based paint. Ventilate the room by placing an exhaust fan into a doorway or window opening and place a respirator over your mouth and nose to avoid inhaling paint fumes.