Blog Post

Scraping the Surface of Exterior Paint Prep

For wood siding, preparing the surface is as important as the paint itself. Here are some factors to look for, or fix, to help that next paint job last.

Premature paint failure is often caused by poor preparation.

Even before you choose an exterior paint product, it's important to learn a bit about what makes paint stick--or not. For background I sought out a few paint prep tips from an expert, Bob Cusumano, president of Coating Consultants and past president and current technical director of Painting and Decorating Contractors of America.

Get the lead out

According to Cusumano, "You first have to consider whether or not there is a previous coat of lead paint." If you have a house that was painted before 1978, there is a good chance that there's lead in the paint.

Lead can cause serious neurological and other health issues, especially in the young, but "a lot of architects don't understand the implications of lead regarding cost and the required steps in the preparation process," he says. If there is lead paint present, there are significant environmental and legal ramifications, and up to three times as much cost.

Lead removal is a complicated issue, so look for more information from us in the future, and from EPA's guidelines. For now, let's assume there is no lead.

Some prime considerations for old clapboards

What is the integrity of the wood surface? Wood with rot caused by exposure to moisture, the sun's ultraviolet (UV) rays, pollution, or other factors will not hold paint well and will need to be cleaned, repaired using an epoxy filler, or replaced.

In some cases there, might even be rot lurking underneath reasonably sound paint. Cusumano recommends probing the surface of the wood with a small sharp knife. It will sink in if there is rot underneath, and the paint won't last long.


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But before replacing the wood and repainting you should figure out what caused the damage. Problems such as poor flashing that allow moisture in behind the clapboards should be fixed or you'll run into even larger problems down the road. Mold or mildew need to be cleaned off, too, and the area thoroughly cleaned. If not properly treated, the fungi will just grow back through the paint, and if not properly rinsed, the paint will blister off.


  • How well are the existing coats adhering? "Every coat of paint you apply over what is already there applies more weight and stress," said Cusumano. Adhesion tests will determine if the coatings are going to hold. If the paint is ok, then you can go over the top. If not, the paint has to be removed, hopefully through simple scraping. An adhesion test, where a paint is applied, allowed to cure, and then tested, is worth the time and can avoid expensive paint failures and callbacks.
  • Is there chalk on the surface? According to Cusumano, "When paint ages and the resin breaks down, the pigment is released as chalk." Paint will not adhere to chalk, so it has to be removed or treated. Depending on the thickness of the chalk layer, "You have to wash it, scrub it, or use a primer that absorbs into the chalk and solidifies so you have a sound base for additional coats."
  • How glossy is the paint? New paint will not adhere to a glossy surface either, so it will need to be sanded to provide mechanical adhesion for the next layer.

What about new wood?

New wood siding can have two very different surfaces. Siding that comes out of the mill with a rough-sawn appearance will need to be sanded, or the paint will have a difficult time penetrating the rough surface. Siding can also have the exact opposite condition, an ultrasmooth surface called mill glaze.


Paint-shaving tools like this one can remove layers of old paint and can be hooked to a HEPA vacuum as part of a lead abatement process.

Cusumano explains, "Mill glaze can happen through the planning process as the resin in the wood gets heated up and comes to the surface creating a plastic-like finish on the top." For mill glaze, you need to do some heavy sanding to abrade the surface.


The sun is not your friend

Peter Yost, BuildingGreen's residential program manager, points out that ultraviolet (UV) radiation can damage the top layer of wood after only a couple days' exposure, compromising the primer's ability to adhere to the wood. For new siding, a factory-applied primer can take care of this problem, but for a repaint job, the surface has to be properly scraped or sanded to expose a new layer of wood (or well-adhered paint layer), and contractors need to move quickly to get that first coat on.


Some cool tools

We list a paint-shaving tool in GreenSpec that uses replaceable tungsten carbide blades whose depth can be adjusted to strip even the thickest layers of paint. They can be set deep enough to strip off a thin layer below the paint, exposing fresh wood that is more likely to accept paint. But the best part is the tool can be hooked to a HEPA vacuum, helping to simplify the complex work of lead remediation.

For other systems and tools that help with lead paint removal and abatement, see GreenSpec's listings under CSI Section 02 80 00: Facility Remediation.

Hire a competent professional

Getting paint to stick to wood siding is a complicated business, and even with the best prep, a paint job will not last forever. Hiring a qualified professional with years of experience painting in your specific climate will go a long way toward increasing your paint's service life. 

Published February 16, 2012

(2012, February 16). Scraping the Surface of Exterior Paint Prep. Retrieved from

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February 19, 2012 - 6:15 am

Hi Robert,

There are certainly other issues that can lead to paint failure, and moisture is one of the big ones. As you mentioned, moisture from inside the house can be a significant problem, especially with old clapboards with no primer on the backside. Other problems include, lack of drainage behind the clapboards (often caused caused by overpainting and sealing up moisture pathways) and microcracks in old layers of paint…the list goes on. Perhaps I should have been more explicit, but I lumped these under the “before replacing the wood and repainting, you should figure out what caused the damage” caveat. Flashing is just the most obvious example.

I spoke with Cusumano regarding rough sawn wood before publishing this. Yes, it does have more surface area, but it is more difficult to completely cover the surface, which will lead to trouble down the road. You can minimize the risk by using a brush rather than a sprayer to work the paint into surface, but if you have air/moisture behind the paint, you will get cracking eventually, which will then lead to more moisture penetration and eventual paint failure.

Dirt and pollution are big concerns, of course, but I included UV damage because it is less well known. I love the look of weathered wood and some cedars will last a long time without a treatment (much longer with), but the UV damage data is not conjecture on Peter’s part. This information comes from research done by Sam Williams at the US Forest Products Laboratory. You can get a much more nuanced perspective from the primary source along with electron micrograph images of the damage.

Again, this is just an overview of an extremely complicated topic, but there will be more exterior-paint-themed articles in the future, so I look forward to more feedback.

February 16, 2012 - 8:13 am

It's important to note that most cases of blistering or wholesale peeling of paint are caused by moisture migration from inside the house, with the vapor pressure literally lifting the paint film off the wood substrate.

I have to take issue with the statement that rough-sawn siding needs to be sanded for the paint to be absorbed. Rough-sawn wood is chosen for its rustic appearance and sanding would undermine that value. But, more importantly, rough-sawn wood has far more surface area and "tooth" for both paint absorption and paint film adhesion and needs no prepping other than possibly sealing knots on resinous species with a shellac primer like BIN. It takes and holds a finish better than planed wood.

I also will take issue with Peter Yost's statement that UV will degrade new wood in as little as two days. UV degradation that is significant enough to cause absorption or adhesion problems is visible as a greying of the wood surface and can takes months to years. Of more significance is air-borne particulate pollution that can sufficiently coat the wood surface to interfere with paint adhesion. This is why the standard recommendation is that all new siding and trim be finished within 30 days of application (or sooner in dirty environments).