You do not need to prime painted walls as long as the existing finish has the same base as the fresh paint; however, if the base is different, you will need to add a specific type of primer that will ensure compatibility. Learn when you should and shouldn't add a primer base coat, or you could end up with peeling.
Compatible Paints
If the wall is already coated with a water-based latex or acrylic paint, you don't need to prime it before applying a fresh coat of acrylic or latex paint. Because each of these paints is water based, compatibility problems will not occur. Likewise, if the wall is already finished with an oil-based paint, you may apply a fresh coat of oil-based paint without priming first.
Incompatible Paints
If you try to add a water-based acrylic or latex paint directly over a wall that has already been finished with an oil-based paint, peeling will result. The same will hold true if you add an oil-based paint directly over a wall finished with water-based paint. Because oil and water are incompatible, these paints will not bond.
Primer
The only way to apply paint over the top of a wall already finished with an incompatible paint is to prepare the surface for adhesion by adding a special primer. Ordinary latex, acrylic and oil-based primers aren't suited for this type of project because their bases prevent them from bonding to both types of paint. For a durable finish that won't peel, pretreat the wall with a shellac-based primer that will bond to latex, acrylic and oil-based paints.
Considerations
Though you do not need a primer if the two paint bases match, you may still have problems with adhesion if you don't take steps to prepare the wall for its new finish. If the wall is coated with a semi-gloss or gloss latex, acrylic or oil paint, it may reject the fresh paint because the sheen could prevent adhesion. Instead of priming, gently scour the wall with sandpaper to roughen the existing paint, which will help the fresh layer of paint bond and prevent possible flaking.
