Unpainted Walls

• If a wall has never been painted before, a coat of primer will help seal out moisture, create an even surface, help hide imperfections, and prevent the paint from soaking into the wall. It will also help make the color appear even instead of blotchy and reduce the number of coats needed to get a professional look. A single coat of primer is all that is necessary to prepare a new wall for painting.
  
  

Damage

• If the wall has been damaged and repaired or the wall has been stained or marked up, a coat of primer will help make it look new again. The primer can smooth and seal repaired surfaces, and cover over stains and marks. It will also prevent the stains and marks from bleeding through the paint. Like with a new, unpainted wall, only one coat of primer is necessary to create a good base for the paint.
  
  

Color Changes

• Primer is not a necessity in the case of drastic color changes, but it can reduce the number of coats of paint needed to make the change and help ensure that the final color is even. If the color difference is more than three or four shades, consider applying a single coat of primer over the old color before painting. This will give the new paint a neutral starting point and it will take less paint to cover the wall.
  
  

Paint Type

• Primer is necessary when switching between oil and latex paint. A special binding primer should be used so that the new coat of paint will adhere to the wall. Applying latex paint directly to an oil-based paint can result in uneven color and coverage, and the paint chipping or peeling away in a short period of time. A coat of bonding primer will stick to both the latex and oil-based paint and keep the paint on the wall.