Even though it is one of the most commonly painted substrates, wood can present some problems, even for the experienced pro.

Wood is among the most common and, in many ways, among the most difficult substrates you paint, because compared to other common substrates, such as stucco, it is the most unstable.

The natural water retention characteristics and variations in porosity of wood cause it to swell when exposed to moisture such as humidity, dew and rain, and shrink when drying out. In turn, these characteristics can lead to exterior paint failures such as blistering, peeling and cracking.

Because of the inherent instability of wood, two of the prerequisites crucial to long-lasting performance of a quality exterior paint job are proper surface preparation and appropriate paint selection.

To help guide you in these matters, PQI offers the following advice concerning four different job situations: bare wood, pressure-treated wood, a sound repaint surface and an unsound repaint surface.

Bare Untreated Wood
When painting bare untreated wood, begin by thoroughly sanding any unprimed and unpainted wood that has been exposed to the elements for more than a few weeks. This includes new wood used for new construction or repairs, as well as old “gray” or weathered wood.

PQI tests clearly show that the adhesion of primers, paints and stains can be compromised by the weathering of wood prior to the application of the coating. Thus, a thorough sanding before priming is essential to a long-lasting paint job.

Also sand flat any shiny “mill glaze” that may be on new wood. Thoroughly dust off any sanded wood with a brush, and then remove all dirt and mildew. Finally, prime all areas to be painted.

When choosing a primer, remember that wood may contain tannins that can bleed through the paint, especially latex paint, resulting in discoloration. To help prevent this, it’s generally best to apply a stain-blocking exterior wood primer. Both latex and oil-based primers are available for this purpose.

Certain kinds of wood, such as cedar, redwood and mahogany, tend to contain high levels of these tannins. For particularly bad boards or sections, use an oil-based stain-blocking primer.

Priming under humid conditions, priming damp wood, and priming at less than the recommended spread rate will increase the chance of stains coming through the primer. If the first coat of primer shows significant surface staining, apply a second coat.

Primers are not designed to withstand direct exposure to the weather for any extended length of time. As a result, it’s important that you protect the primer by painting it within a week, or according to the primer manufacturer’s directions.

When painting, remember that top quality exterior latex paint generally may be applied to an oil-based primer, but it’s wise to check the primer and paint manufacturer’s recommendations first. Use quality exterior latex stain-blocking wood primers only with latex finish coats, never with oil-based paints.

Pressure-Treated Wood
Prepare pressure-treated wood the same as other bare wood. However, if the wood still contains moisture from the treatment process, allow the wood to dry open to the air for two to four weeks before coating, depending on dimensions. This is especially important if using an oil-based paint, which can otherwise lock moisture in and lead to rotting.

When painting treated wood, apply one or two coats of a quality exterior wood primer. For the best durability on treated wood, apply two coats of a top quality exterior acrylic latex paint over two coats of quality primer. While either a latex or alkyd primer can be used, a quality latex primer will provide the best long-term crack resistance.

Sound Repaint Surface
A previously painted surface is considered sound for repainting if the existing paint maintains a good, solid, continuous film, i.e. shows no signs of cracking, peeling or loss of adhesion. If this is the case, then begin by removing all dirt, chalk and mildew.

Next, check old painted surfaces for glossy areas. Dull these surfaces by sanding, wire brushing or using a liquid deglosser. This is especially important on areas protected from weathering, such as under eaves. Then wash and rinse thoroughly.

Medium-grit garnet sandpaper is suitable for sanding bare wood prior to painting, but sanding marks may appear in the finish coat if it is used for dulling a glossy area, particularly if a semigloss or gloss finish is to be applied. It is better to use a fine-grit garnet paper when dulling gloss.

Priming is usually not necessary if the old paint is in sound condition and is still adhering well. However, the use of an appropriate primer on a sound repaint surface can be very beneficial because it will provide maximum adhesion, maximum mildew resistance, and a uniform appearance of the finish coat.

Unsound Repaint Surface
If the existing paint film is unsound, remove all loose paint by sanding, scraping, wire brushing or power washing. However, if you suspect that lead may be present in the old paint, do not sand or remove the old paint, as this will release lead into the environment. Follow lead abatement procedures required in your area.

Feather-sand any rough edges on the remaining paint; otherwise, new paint will flow over the edge of old paint, creating thin spots that can fail early. Feather-sanding is best done by hand. Aggressive power sanding can generate heat that can weaken the adhesion of the original paint.

If you remove old paint completely and expose bare wood, sand the surface to expose fresh wood fibers, and then prime. For best results, prime the entire job.

Latex Over Oil
A top quality exterior acrylic latex paint is an excellent choice when repainting. And it can be applied with confidence over oil-based paints, although some latex paint manufacturers do recommend use of a primer.

One situation to be aware of, however, is that of a heavy buildup of old oil-based paint that is shrinking with age. A coat of latex paint can apply enough stress during and after drying that it can lift the old paint from the surface. In cases where there is such a buildup, continue with an oil-based paint.

On the other hand, it is best not to apply oil-based paint over a latex paint or primer, since the latex product can be considerably more flexible than oil-based paint. As a result, the oil-based finish coat may have an excessive tendency to crack with time. An exception is the application of alkyd primers over latex paint, because they are formulated to remain more flexible than are alkyd finish coats.

Wood differs from most of the other substrates you paint because it is a natural substance. Yet, it continues to play an important role as both a structural and decorative material. By knowing how to paint wood properly, you will allow this natural wonder to continue to perform its unique function for years to come.

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