Interior paint bubbles, also known as blisters, occur when paint separates from the wall and swells, forming a bubble. Bubbles appear most often with new paint but existing paint can bubble too. Both water- and oil-based paints are susceptible. Bubbling is not caused by bad paint. It results from environmental conditions where the paint was applied.

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Warm Walls and New Paint

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Sun-warmed walls are at risk for bubbling. Normally, paint solvent evaporates gradually as paint dries. When the wall is hot, the solvents in new paint – water in latex paint, mineral spirits in oil-based paint - begin to vaporize as soon as the paint is applied. At the same time, the paint begins to harden, forming a vapor-resistant film. The vapor gets trapped under the film and the paint swells, causing it to bubble. (reference 1: "Warm or Hot Surface", para 1)

Walls that have been painted recently and then begin to warm rapidly from sunlight or proximity to a heating vent are also susceptible to bubbles for the same reason. Temperature bubbles may occur within hours of painting or may take several days to appear. (ref 2: "Blistering", para 1)

Damp Walls

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Moisture in the walls also causes new or existing paint to bubble. (ref 2: "Blistering", para 4) Moisture can seep into unsealed walls when the wall is below grade or is made of a porous material like concrete block. (ref 1: "Wet or Damp Surface" para 1) It can also occur suddenly due to a plumbing failure or roof leak. When the water migrates from the wall to the interior of the room it must pass through the paint. The pressure makes the paint separate from the wall. (ref 3: "Possible Causes")

Excessive Humidity

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Drying paint will bubble if there is excessive humidity either during paint application or after the paint has started to dry. Bubbles fill with water absorbed from the moisture in the air. (ref 1: "Rain, Dew and High Humidity" para 1) Even recently applied paint that has already dried can bubble from excessive humidity. Poor ventilation contributes to the problem. (ref 3: "Possible Causes")

Incompatibility with the Underlying Surface

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If the previous paint is incompatible with the new paint, it's likely that the wall will bubble. Surfaces that were originally painted without a primer or were painted first with an oil-based paint are suspect. The old coat of paint may not lift until the new paint dries fully. As the paint dries it shrinks, causing the unstable surface underneath to separate from the wall and bubble. (ref 1: "Lifting old layers of paint" para 1 and 2)

If there are contaminants on the surface, the paint won't stick. It might blister or crack after it begins to dry. (ref 3: "Possible Causes") For instance, residual wallpaper glue causes paint bubbles.

Finally, a defective undercoat such as primer that has frozen and thawed will reject paint. It might bubble or dry and then split open.