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.
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.

 

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