Bare plaster walls must be sealed before they receive a top dressing of paint or wallpaper. Plaster is quite porous. And when paint or wallpaper adhesive is applied to unsealed plaster, the pores soak up a considerable amount of the moisture making the job much more difficult. A sealant fills and seals the plaster's pores allowing the next coating to stay on the surface. Plaster sealants and combination primer-sealants are sold at most hardware stores and are applied like paint.
Mix the sealant with a paint mixing stick.
Pour out enough sealant to fill the small reservoir in your paint tray.
Dip your paintbrush in the sealant and paint a 2-inch border around the plaster wall. Cover the areas above baseboards, around fixtures and windows and any other spots that cannot be efficiently covered with the broad strokes of a paint roller.
Roll the paint roller in the sealant so that the roller is well coated but not dripping with paint. Use it to completely cover all surfaces of the wall.
Allow the sealant to dry for the manufacturer-recommended amount of time -- usually overnight.
Apply a second coat of sealant if desired.
Be sure to prepare the room before you paint. Lay down plastic sheeting along the wall to catch any drips. Apply painter's tape along the baseboards, around window frames, light fixtures and anything else you don't want to accidentally apply sealant to. Open the windows and/or turn on the fan to increase air circulation in the room.