Bonding primers are thick, heavier primers used when additional material is going to be applied over the plaster walls. Occasionally, more plaster is added for a heavier textured wall. The bonding primer ensures adequate adhesion between the current plaster layer and any material covering it. The bonding primer does not increase adhesion between the current plaster and any underlying material. These primers should be applied quickly, as they tend to dry fast.
Some primers prevent underlying stains and odors from seeping through. The stains can be created by old paint, pen marks or mildew. Odors may have been left by smoke or pets. Often, the sealer and odor blocking primers leave heavy odors themselves, until a few days after drying. Always work in an environment with excellent air circulation. Keep windows open and take frequent breaks. Wear a breathing mask approved to decrease inhalation of paint odors.
A basic paint primer is often still recommended, even when you're simply painting the plaster, and no stains or odor are present. The primer ensures your topcoat of paint will look uniform and even. You may even have to apply fewer coats of paint. Use dark gray primers when painting in dark colors such as red, burgundy, navy blue or green.
Primer is applied the same way as a coat of paint. Roll it evenly onto the wall in a "V" pattern. Apply it in vertical sections. Use an angled brush to apply paint to the edges of your plaster wall, where paint can't be applied with a roller. Allow it to dry according to the manufacturer's instructions.