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blistering paint

I painted the cedar siding last year. Now there are many areas that the paint is blistering. It looks like there is water in the blisters. The siding had been painted many times before. I scrapped amd sanded the old paint. The guy at the paint store told me is is trapped water behind the paint. How do I get the water out??? Please help! I am getting mixed to no answers from the contractors here. I live in Pittsburgh Pa.

Re: blistering paint


You didn't say what you primed and painted the siding with. Also, you did not state how old the house is and if it has had a history of peeling and blistering?

Older homes often lacked good vapor barriers in the walls just behind the plaster or drywall. Often insulation is also minimal, especially before the first "energy crisis" of the early 70's, after which standards were raised. Every house generates moisture inside through showering, cooking, humidfiers, even simply breathing ( a human exhales a gallon of water per day!).
This vapor will try to exit to the outside right through the walls, especially in winter when the relative humidity can be zero outside. If nothing stops this moisture from getting into the walls, the next resistance to its transfer is the backside of your siding. With wooden siding, it was the practice in years past to prime the back side of siding with an oil based primer to prevent moisture from entering into the siding. If it gets into the siding, the heat of the sun will raise the vapor pressure in the wood (steam) and blisters and peeling will result.

Now, what can you do about it? First, try to lower the moisture within the house. Install good exhaust fans in the kitchen and bath to remove high humidity. Lower the humidifier in winter if you have one. If your house is on a crawl space and there is no vapor barrier on the ground, cover it with 6 mill plastic. As you repaint your rooms, paint all the exterior walls with a primer with a high perm rating to prevent moisture from entering the walls.

Now, what to do on the outside? How much paint has accumulated on your exterior siding? Every additional coat of paint reduces the ability of the house to breath. It becomes a vapor barrier on the outmost surface - the worst place! At some point, stripping the paint is a consideration, or outright replacing the siding.

In terms of breathabilty, acrylic paints are much more permeable than are oil paints. They have the ability to let vapor pass on through without bursting the paint film. Acrylics form a much more flexible film which can expand and contract with the heat and humidity swings of the seasons. Oil paints form a more brittle film and become more so with age.

Generally, acrylics are considered the preferable housepaint. However, if your house has had many coats of oil paint over the years, there is a danger that a coat of well bonded acrylic paint, which is expanding and contracting and the oil paint is not, can actually break the bond of the oil paint to the siding and massive peeling might occur. In this scenario, it might be advisable to stick with the oil paint.

Re: blistering paint

i hope you used a good primer before you painted. if not you need to resand it and start alover with a good primer, mabe two coats. then paint .

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