Primer Primer

September 17, 2005 11:28 AM

How is using a coat of primer before painting different than just using an extra coat of paint?

We just had our fifty-year-old house painted, and, unfortunately, did not get a detailed breakdown of what would be done. They power wahsed the worst of the old paint off, and did a little scraping, but then just put the paint straight on the resulting surface, with no primer. Is this bad?

Everything I could find on google stresses the importance of starting with primer, but I'd like to know how that's different from just using an extra coat of paint.

posted by notbuddha to home & garden (11 comments total)

1. Primer is cheaper.

2. Primer is usually matte paint - better adhesion, and easier to overcoat.
posted by Meatbomb at 11:43 AM on September 17, 2005

Two coats of "real" paint will be more expensive for the contractor, no real difference to you the customer whether it's primer and paint or two coats paint.
posted by Meatbomb at 11:45 AM on September 17, 2005

In situations like this where there is power washing and an assumed difference of base materials to be painted over (different coats of pain, perhaps some bare wood or other surface, etc), the use of primer is very important to make sure the finished result looks even in both coloring as well as texture appearance.
posted by FlamingBore at 11:54 AM on September 17, 2005

There are several variables in play that will change the response to this question. First and foremost, it depends what was the underlying painted surface. If it was wood, and the scraping exposed bare wood, then it is extremely important to have primed the bare spots.

In addition, it depends on what kind of paint was on the still-painted areas. If it was oil/alkyd paint, you should know that acrylic latex is much more popular than it was even 15-20 years ago - but latex doesn't cover alkyd very well. So if your painter used latex paint on unprimed alkyd, you have a problem. Primer should be used to bond to the alkyd and provide a surface to which the latex can bond (though even in that case many painters would recommend sticking with alkyd).

The trick is that primer isn't simply cheap paint. It has different properties, including the way that it bonds and is absorbed by different materials. In doing so, it provides a uniform surface to which paint can bond, which prevents different kinds of absorption on different areas of the house and so allows the paint to form a uniform layer in all areas. This is significant aesthetically (in that areas where paint absorbs will look different than areas in which it didn't absorb) but more importantly is significant in terms of maintenance. Areas where the paint doesn't form a uniform layer will weather differently than other areas, which will eventually mean you'll have to paint sooner.

If it is another surface, not bare wood or painted wood with bare spots, then things are different, and I don't know the specifics.
posted by mikel at 12:06 PM on September 17, 2005

Primer is often thicker. Primer is more reflective of light (light will penetrate paint and be reflected back through it). As mikel says, if you're laying water-based paint over oil-based paint, you need primer between the two to get it to stick.

As an experiment (I've done this), take a piece of wood or metal and paint part with two layers of paint, and another part with one layer each of primer and paint. The difference will be obvious: the color of the primer+paint will be much more vivid and uniform.
posted by adamrice at 12:50 PM on September 17, 2005

More important than the primer in your situation is, how many days did they let the wood dry after power washing it? If you say less than 2, and you don't live in a desert, you will need to repaint in a few years.
posted by 517 at 12:51 PM on September 17, 2005

Primer is not just cheap matte paint, it is a chemically different formulation and it really does help adhesion and coverage over unpainted surfaces and over those that have been painted with a different type of paint. In other words, what mikel said.
posted by kindall at 1:37 PM on September 17, 2005

Yeah, what the others said. If they scraped down to bare wood and didn't prime, you may be in trouble.
posted by blag at 4:57 PM on September 17, 2005

The answer, of course, is....it depends. There are different schools of thought on latex over oil. (Though oil over latex is ALWAYS a 'no-no'.)

The difference is going to involve:

-how many layers of paint are on there to begin with?
-are those layers oil or latex?
-how did they prepare the surface? Did they scrape to the bare wood? Or-did they at least sand the top coat?
-what is the quality of the new paint to be applied?

Paint can fail in many ways and for many reasons.

I have a copy of an article from a back issue of Fine Homebuilding which is the "be-all and end-all" of exterior paint info, IMHO. You can purchase the same article online. One of those sources of information that is worth every penny.
posted by jeanmari at 5:24 PM on September 17, 2005

I recently encountered this same problem. The guy at the hardware store said "Think of primer as glue, not as paint." In other words, it's a glue that sticks to wood and other surfaces, and in turn is something that paint sticks to. So, yes, there can be an enormous difference between primer + paint versus two layers of paint. [Your basic question is whether the first layer of paint will stick to whatever was under the old paint, wherever that whatever was exposed.]
posted by WestCoaster at 5:36 PM on September 17, 2005

Thanks for the answers, folks.
posted by notbuddha at 9:28 AM on September 19, 2005