Primer is white paint that's specially formulated to provide a clean, even base for your topcoats of paint. It makes the paint look more even, because you don't have color variations on the surface below it. For problem surfaces that have deep stains or stark color differences, get a sealing primer, which provides a chemical seal in addition to merely a layer of white. Sealing primers come in exterior and interior forms and should be used in ventilated areas. Make sure your primer is the same type of paint--oil or water-based--as your topcoats.
A good primer covers up a lot of surface problems but not all of them. The surface you're painting still has to be clean and free of dust, dirt, grime and gloss. Clean the surface with a household cleanser and let it dry thoroughly. If you're painting a plaster or drywall wall, use patching plaster to patch holes and cracks, and sand it smooth. Scrape away any peeling paint. If the surface is glossy, use a belt sander with 120-grit sandpaper to buff it flat.
Taping and Tinting
Tape off the borders of the area you want to prime, using painter's tape rather than masking tape. Painter's tape comes off easier. For an interior wall, put the tape along the edges of trim, the floor and the ceiling. Tape up any fixtures that can't be removed. If you're topping the surface with dark paint, open your primer and mix in one cup of the paint. This will "tint"' the white primer toward the color of the paint, making it easier to cover.
Apply the primer as you would apply a thin layer of paint. Start by cutting in around the edges with a brush, working from the highest point downward. Make sure the brushlines are smooth and flat, with no dripping. For walls, use a roller. Roll it in the primer, then roll the primer on the wall all at once in a few long up-and-down strokes. Then work the primer across the wall with a series of long, overlapping "M"-shaped strokes until it thins out and is even. Allow the primer to dry for an hour or two before applying the paint.