Paint primers serve several purposes

on January 02, 2008 at 4:50 PM, updated January 02, 2008 at 4:53 PM

Q. The instructions on the can of paint I am getting ready to use say to use primer paint on bare wood. Is that really necessary? It seems like a waste of time.

What are the benefits of using paint primer? I feel the paint company just wants another sale and they are trying to slow me down. I have never used primers before, and the paint I use seems to hold up. If you do convince me, is there a special primer I should use?

A. I am sure the things you painted before survived the lack of primer, but I am here to tell you that the finished product would have been much better had you used one. Paint primers are very necessary, and the serve several very important functions.

A paint primer is a special paint. It is formulated to do several mission-critical things, the least of which is create a superior bond to the object being painted. Keep in mind that most paints are really glues with color in them. In other words, when applied correctly to the right things, paint can stick or adhere to something for a very long time, just as any glue you would apply out of a bottle.

The chemists who work for paint companies formulate primers so they are very sticky. The grab onto surfaces like briars to your pants. What's more, the dried surface of many primers is formulated to bond physically and chemically to the finish paint if you apply the finish paint within 24 hours of applying the primer.

Certain primer paints also serve two other functions; they seal a porous surface and they even out the texture of the object so the finish paints think they are being applied over a sheet of glass.

If you were to look at the surface of the wood you are about to paint under a microscope, you would be surprised at how rough it really is, even after you have sanded it. What's more, depending upon the species of wood you are painting and its grain pattern, you can have widely varying levels of porosity between the light-grained springwood and the darker grain bands of summerwood.

If you were to paint over wood without a primer, you might be able to see these grain bands, as the finish paint would look dull over the light-colored springwood but glossier over the darker bands of wood. This happens because the paint soaks into the light-colored springwood. Primer-sealer paint seals the surface of wood so the finish paint does not soak into it. This allows you to have even gloss over the wood that is painted.

Many paint primers for wood have lots of dense pigments in them that act as microscopic liquid-spackling compound. The pigments fill in all of the low spots in the wood so the finish paint will look like satin.

Modern primers are often water-based and dry very rapidly. Some can be recoated with finish paint in less than an hour. Using a primer will actually speed up the job, not slow you down.

I urge you to always use a primer if the label on the paint can recommends it. In fact, pay attention to what primer they say to use, as the primers are often formulated to work with specific finish paints.

After painting bare wood with a water-based primer, the wood may seem rougher than before you started. This frequently happens for two reasons; the water in the paint can raise the grain of the bare wood and you feel some of the coarse pigments in the paint. For truly professional results, you need to lightly sand the primer before you apply the first coat of finish paint. The primers sand very easily, so it will not be a chore to do this task.

Try to recoat the primer with a finish coat of paint within 12 hours, if at all possible.
