Wednesday, September 19, 2007

Low temperature House Paint

Cold weather exterior painting is always a dilemma. We have fought this issue for years in our market. Painters love oil for the low temp tolerance, and also because it is easy to get a smooth finish with slow dry oil paint. On the downside, oil based house paint will always eventually lead to hard brittle paint that chalks and cracks. We usually see some chalk, and cracking at joints within 1-2 years. Moisture that is under paint film, either from interior moisture migration (on window sills), or water that enters through cracks at joints, will be trapped under the paint film causing microscopic wood decaying organisms to thrive. Mildew and fungus just beneath the paint film will cause the paint to release along with a very thin layer of damaged wood fiber. By the 3d to 5th year, we will generally see big problems on sills and wood joints, with cracking, and moisture damage to wood. Oil paint never stops hardening, so this type of failure is inevitable, it just varies by existing conditions, and product used. Bottom line, we never condone the use of oil based paint on exterior wood, and only a few special circumstances will ever warrant the use of old technology oil based paint.

There are a few Low temp Acrylics on the market that allow painting down to 35 degrees, but you must have at least a few hours of good temperature. My worry is that painters get the low-temp and assume they can paint untill late in the day, then temperatures drop to the teens at night and the uncured paint is exposed to unfavorable conditions at the most critical time for the paint film. Low temp acrylic can be a viable option if used properly, greatly extending our paint season (painting when 35 degrees to 50 degrees ). I would always pick a low temp Acrylic over oil based paint, but we have done no extensive testing, so most of the low temp applications we are looking at are only a couple of years old, but doing well.

We still favor conventional 100% acrylic house paints applied in the 50-70 degree range) for the majority of work, and use the low temps when absolutely neccessary.Coronado paint makes a low temp line called Maxum M9000. Coronado's 410-11 House Paint Primer also carries a low temp claim. To apply acrylic paint over existing oil that is failing, any existing chalk or mildew or other contamination must be removed from the surface. Clean with TSP and bleach (bleach to kill mildew and TSP to loosen chalk). A top quality bonding primer like Coronado 116-11 will be recommended on oil paint that is still shiny. Any extremely shiny surfaces must be dulled by sanding especially under eaves and areas not dulled by the sun. There are various other specifics to converting from oil to acrylic on an existing house, such as failed coating removal, and caulking issues. Hope this helps.

Doug Wilson

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