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Why does this exterior paint keep blistering?

A reader wants to stop paint on wooden house siding from blistering.
A reader wants to stop paint on wooden house siding from blistering. (Reader photo)
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Q: We have a problem with paint on the front of our house. Previous owners had painted with Duron paint. Last spring, after we had lived there for two years, we repainted with Behr Premium Plus Ultra Exterior Semi-Gloss Enamel. Several weeks later, blisters developed on three small, shady sections. In the fall, a painter scraped those areas, sanded down to bare wood, primed with Kilz oil-based primer and repainted. Blisters reappeared, and again the areas were scraped, sanded, primed and repainted. But blisters formed again. They have sometimes reappeared on the same boards but not in the same places. Recently, another small area has also developed blisters. The rooms behind the siding are bedrooms and a library/office, without any plumbing. Our gutters and downspouts seem to be working well. We called in a paint distributor, who mused about the lack of full sun on the affected areas. But there seemed to be no problem with blisters on the previous owners' paint. What could be the problem, and how do we address it?

Damascus, Md.

A: Blisters form when the top layer of paint lifts from the underlying surface. There are many possible causes. The surface might have been dirty or the weather too hot or cold, or moisture could be moving through the wall and pushing the paint off. Applying oil-based paint over water-based paint can also cause bubbles, but we can rule that out because you bought a water-based product. Moisture moving through the wall also seems unlikely, given what you describe and the fact that the bubbles aren’t reappearing in the exact same places.

You don’t say specifically whether just scraping off the bubbles reveals bare wood, or whether the old paint stays intact until the painter sands it off. If scraping alone is enough, the initial painter messed up, perhaps by painting over gray wood, which has loose fibers at the surface. But if only the new paint scrapes off easily, something went wrong with your paint job.

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Painting weathered wood isn’t the likely culprit because the paint is fine on walls in direct sun, which are most prone to weathering. But persistent shade might be part of the problem, at least indirectly, as the paint dealer mused. Maybe the painter didn’t realize the shady surfaces were dirtier or mossier than the others, or that they were colder. Or did rain hit them within 24 hours after the paint went on?

Some of today’s water-based paints, including the product you used, can be applied when the air, surface and paint are as cold as 35 degrees Fahrenheit, but going by just air temperature can be misleading, as McCormick Paints, a family-owned paint manufacturer in Rockville, Md., shows on its website ( with pictures of an infrared thermometer recording surface temperatures varying between 71 degrees in bright sunshine with no wind to 14 degrees in full shade with strong wind while air temperature stayed steady at 43 degrees.

If your painter worked on the front of your house when the air was close to 35 degrees but the surface got a few degrees colder before the paint dried, that could explain what happened, said Rick Watson, director of product information and technical services for Sherwin-Williams, which bought Duron in 2004. “If you put the paint on and the film is freeze-dried on the surface, the paint doesn’t have a chance to really bite and adhere to the existing paint film,” he said. “Then what happens, if you get a little water or moisture on the surface, where the paint could not adhere to the existing substrate, you get a bubble.”

Whatever the cause, what to do now?

First, if the house was initially painted before 1978, determine whether the paint contains lead and then follow proper procedures to avoid exposing anyone to lead dust.

If that’s not an issue, you have two options: keep scraping, sanding, priming and repainting as bubbles appear, or strip off all the paint that’s bubbling and start over. Watson said that if he were doing this on his own house, he would start by washing the surfaces, not by scraping and sanding as many people assume. “The first mistake is to sand first,” he said, because that just grinds grime into the surface. “I want to wash and degrease first, then scrape and sand and go from there.”

He would get a sharp paint scraper and remove loose paint by hand. “A contractor is going to power wash,” Watson said, but he doesn’t recommend this for most homeowners because too high a pressure damages wood. If all the paint needs to come off, power washing might be enough, or you might need a contractor who uses a chemical stripper, Watson said.

As for repainting, he said, it depends on how long you plan to stay in the house. If you’re fixing it up to sell, he suggests two coats of an exterior paint that doubles as primer. “If it’s my home and I’m going to live there for a long time, I’d root out the cause, strip to bare wood, use oil primer and two coats” of high-quality water-based paint, he said.

A call to Behr’s customer-care number (800-854-0133) resulted in similar advice, except it recommended either water-based or oil-based primer. Even though the Behr product you used is advertised as paint and primer in one, using a separate primer gives the final paint a better surface to stick to, the customer-care representative said. “Primer is always better,” he said. “It can be any primer — latex is fine.” (Latex is another word for water-based.)

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