If you are are painting a water based paint over a water based paint, you probably do not need a primer. The same goes for an oil based paint over an oil based paint. Paint sticks to paint fine. So why do you care if the paint is a “paint and primer in one”, and what does that mean? Is it just an advertising gimmick?
As a painter I find the description that a paint is a “paint and primer in one” misleading. There have been paints around for at least 30 years that were considered “self priming”. This simply means that on new raw surfaces the paint can be applied directly to the existing surface using 2 coats, and the first coat will adequately seal the surface for the second coat. Even these paints usually have a disclaimer that a true primer is preferred, especially under certain conditions. The advertisements really only say that it covers well, so a separate primer coat is not needed. There is not really primer mixed into the paint, because that would degrade the paint. Primers are not intended to be exposed to air or sunlight for any period of time, and will deteriorate quicker than paint if exposed. Paints in most cases are designed to sit on top of the surface and perform as a protective barrier, and primers are designed to penetrate or bond where needed.
So when is a primer needed? Primers are fairly specific in their purpose, and that is why there are so many different kinds of primers. Generally any raw surface or substrate needs a primer, and check with the paint supply store for their specific line of primers, but here are the general guidelines:
- Porous surfaces like raw wood or drywall need primers to soak in and bond, and create an even surface so the paint can sit on top. Painting directly over a porous surface will draw out some of the binders and solvents, changing the final composition of the dry coating as well as affecting the sheen and look.
- Water soluble stains that are activated with water based paints, and bleed through, need sealers.
- Slick surfaces need adhesion primers to really grip the surface.
- Ferrous metal needs rust inhibitive primers to prevent corrosion.
- Galvanized metal needs a primer to grip to the metal.
- Fresh stucco if not properly aged needs a primer to neutralize the alkalinity of the concrete and prevent alkaline burns.
- Existing oil based coatings need a bonding primer as an intermediary coat before using a water based paint. The standard for this kind of primer used to be an oil based undercoat primer, but with new air regulations these are no longer available in California so manufacturers have had to come up with some new water based bonding primers. Some examples are Stix by Insulx and Griptech by Rustoleum.
- A case where you may want to do a full interior wall prime is when there has been extensive minor marks and nail holes etc. such as apartment walls. In that case a full prime or a resurfacing primer can give a clean slate to work on.