How to prevent and repair paint bubbles

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Q: I’m trying to figure out why the paint is bubbling on a ceiling in my house. Could it be sweat on the HVAC duct, which sits above the area where the paint is cracking and peeling? If yes, what is the solution?

A: Humidity is probably the most common reason for paint bubbles and peeling, and it’s also the only possible cause that can lead to long-term structural and air quality issues. So when the paint is peeling and you’re trying to figure out why, start by asking yourself: Could moisture be the cause?

In your case, you have a good idea that the problem could be humidity, and you know where it could be coming from. To confirm your hunch – or as a first step when there is no obvious explanation – use a moisture meter, such as General Tools pin type digital model ($36.97 at Home Depot). Or get a pinless meter or one that switches between pin and pinless, like General Tools Combo Model ($54.97). (Pinless operation is easier for checking a general area; pins are excellent for pinpointing a problem.)

First take a reading where the paint is intact; it will likely be between 5 and 15 percent, depending on the relative humidity in your home that day. Then check the drywall where the paint is peeling. If this reading is significantly higher, check the surrounding areas to determine where the ceiling or wall is damp and where it is dry. You might discover a streak of moisture that is leading to a plumbing leak, a roof problem, or even a drainage problem outside your home. Regardless of the scenario, address the moisture problem before dealing with the painting.

Condensation forms on heating and air conditioning ducts when warm, moist air hits a cold surface. If condensation only forms in one place, there may be a gap of insulation around the ducts or a leak between sections of duct. If you feel comfortable crawling around your attic, you can try to pinpoint the cause by looking for dirty insulation, which shows where mold is growing due to dampness. Replace any missing insulation.

If a duct joint is leaking, apply duct sealant ($8.67 for 10.5 ounces of main stream at Home Depot), which will work better in attic conditions than aluminum tape designed for sealing ductwork. Apply the sealant with a caulking gun, but smooth the bead with a gloved hand, paint brush, or putty knife to make sure the material fills the gap.

What questions do you have about maintaining your home?

If DIY work in the attic isn’t for you, hire an HVAC company to check and repair the ductwork. Also ask the company to make sure the system is properly calibrated with no clogged vents or dirty filters. A company that provides air sealing and insulation services can address general attic conditions that can make condensation worse by plugging spaces where indoor air leaks into the attic and where insulation is too thin to be effective.

If you rule out moisture as the reason your paint is peeling, there are many other possible explanations. Scrape a bubble or two to see if only the last coat of paint is peeling or if the failure goes all the way to the start. You might even find a layer of drywall mud between two coats of paint, a sign of a previous repair.

If only the most recent coat of paint is peeling, the surface may be dirty, preventing the new paint from adhering well, or too hot, causing it to dry too quickly. Indoors, high temperatures are more likely to cause paint problems where the sun shines on a wall; the overall ambient temperature may be fine, but the surface may be well above the 85-90 degree maximum listed on the paint label. Paint can also bubble when someone paints directly over new drywall or a piece of drywall, without priming first. Drywall absorbs so much of the first coat of paint that the next coat can’t adhere well.

Whatever the cause, preparing for new paint begins with scraping away bubbles and wiping the surface. If it was dirty before the last paint job, use soapy water, then plain water to clean it. Greasy walls and ceilings can be huge problems in kitchens. Let the surface dry, then inspect it. If it is not smooth enough to repaint, apply a thin coat of lightweight drywall mud, let it dry, then smooth it with 100 or 120 grit sandpaper. Or, to avoid creating dust, you can use a damp, hard sponge that you rinse frequently in a bucket of water and wring well. Clean off all sanding dust or wait for the wall to dry. Then prime the surface, wait the time recommended on the label and repaint.

If moisture was the culprit and you fixed that problem, follow the same procedure to prep the surface for painting. But use a stain blocking primer to make sure stains from the previous leak don’t migrate into your new paint.

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