Knowing when you need to prime your walls before painting and when you can do a professional quality job without priming can save you both time and money. When you do use a primer, make sure it is the appropriate one for your particular paint job.
1. Bonding primers create an adhesive bond between a hard, slick surface and the final coat of paint. They are usually oil based or pigmented shellac. Use a bonding primer when painting with latex over glossy oil-based paint or when painting over any wood or vinyl panelling. A bonding primer is not necessary when using latex paint of any sheen over glossy latex paint; however, you should use two coats of your final paint if it is a different sheen than the existing surface.
Stain Blocking Primer
2. Some primers are used to block bleed-through from water damage and tannin staining from cedar and redwood. Often they have the same qualities as stain-blocking primers. A good example is Kilz Original pigmented shellac primer. These primers are also effective at sealing crayon, smoke, magic marker and many odors. For instance, a stain blocking primer will seal both the discolouration and the smoke odor from rooms that were occupied by a heavy smoker.
3. Drywall primer creates a uniformly absorbent surface for the final coat of paint and should always be used as a first coat on new drywall or large drywall patches. It is inexpensive and high-hiding (meaning it will hide the gray or green color of the underlying wallboard), and it sets the stage for a more durable, washable finished surface.
4. Some tinted primers will reduce the number of coats needed for red and other strong colors. Ask the paint store representative whether you need a color primer for your finish paint. It depends on the particular base and colorants used. Tinted primers are often gray.
5. A misconception is that dark walls require primer before being painted with light finishes. Most whites, taupes, beiges and off-whites will require just two coats of paint over even the darkest colored surface. Even if you use a primer, you probably would still need two finish coats.
Should you really want to use a white primer to cover a dark surface, avoid all purpose primers because they are often quite thin. Ask at the paint store for a high-hiding primer.
6. Any time you are changing the sheen of the wall paint–such as from satin to flat or from semi-gloss to eggshell–you will need two coats for uniform sheen. Since most primers have a flat finish, put on two coats of your final paint (unless it is a flat paint).
You should never need more than one coat of primer.