New plaster is porous, meaning it can easily absorb liquids or moisture if it is not sealed. Proper application of primer to a plaster wall makes a surface that can better resist damage and moisture, and allow paint to adhere to the wall successfully. A raw plaster wall will take quite a bit of primer and the job can be time consuming, but it is the most important step in preparing the plaster wall for paint.
Allow the plaster to cure and dry completely. Depending on the conditions of the room where the plaster was applied this could take from 3 to 6 weeks. Moisture may condense on the windows of the room while the plaster is still wet, demonstrating the moisture content of new plaster.
Cover the floor with dust sheets. Tape plastic sheeting over windows or trim to protect them from the primer, using the painters tape. Make sure the tape does not stick to any of the plaster that will be primed.
Dust the walls down with a clean dry cloth. Remove any dust from the walls that will interfere with the primer.
Mix an emulsion of primer to coat the plaster. Plaster will absorb a large amount of the sealant, so buy ample supplies before beginning the job. A thin enough mix to absorb into the plaster and seal it is important, so though it seems odd to add moisture to the primer, it needs to be thinned. For water based materials such as latex thin with one part water to four parts primer. Prepare your mix in the bucket using a regular stir stick or a drill with a paint mixing attachment.
Attach the roller cover to the roller, and if you are starting at a high place on the wall attach the handle extension by screwing it into the end of the roller handle.
Place the bucket screen in the bucket with the primer. The bucket screen hangs off the side of the bucket with the screen portion inside and is used to clean excess product off the roller cover so it is not over saturated. Dip the roller into the primer and roll it back and forth over the screen to distribute the primer evenly.
Begin rolling a small area about 4 feet by 4 feet. Work the primer into the plaster, re-dipping the roller into the primer if necessary. In some cases the first application may almost completely disappear as it is absorbed into the plaster. Continue applying until the plaster is saturated in that area and then move on to another section. Repeat this process until the entire wall is covered. Allow the primer to dry.
Check the surface of the dry primer to see if the dry sandy surface of the plaster is evident. If the primer creates a good sealing layer over the plaster, you can continue on and apply paint. If the surface still seems dry and porous, repeat the priming process. Do not thin the mixture on the second application.