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STUCCO PAINT FAILURES
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Here we explain how and why painting a building exterior in hot sun or when there is a strong wind can lead to early paint failure, blisters, and other paint problems. This building paint failure article series reviews common building exterior & interior painting mistakes, describes how to diagnose paint failures on buildings, and outlines a procedure for diagnostic field inspection & lab testing of failed painted surfaces. We include photographs of paint failures on buildings and more photos of forensic paint laboratory examination of samples of failed paint useful to assist in diagnosing the probable cause of each type of paint failure. Readers should also see Paint Failure Case Photographs-SITE and then Paint Failure Case Photographs-LAB, and see PAINT & STAIN GUIDE, EXTERIOR for a guide to the selection and proper application of paints and stains on exterior wood surfaces.
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Painting in sun and also painting over a primer or base coat that has not adequately dried can cause solvent blisters that are hard to see with the naked eye but are easily identified by microscopic examination.
Thermal blistering, or "temperature blistering" occurs when painting in sun, or if paint is applied to hot surfaces; the blister may be from moisture or solvents in the paint itself. Paint Solvent blisters are small, usually microscopic - you won't see them by naked eye. Both thermal blistering and solvent blistering may occur on the same surface. See PAINT FAILURE DICTIONARY we discuss the details of these paint failure mechanisms. Our photos, below, show solvent blistering and cratering.
Depending on the chemistry and intended application of a particular paint, if applied to a too-hot surface (perhaps over 80 degF) the paint may form a skin which retards evaporation of the remaining carrier vehicle or solvent in the underlying paint. A possible result is the formation of paint blisters, some of which may rupture to form pinholes in the coating surface. A second possible result of painting on a too-hot surface is the formation of cracks in the painted surface where blisters have lifted and thinned the paint coating.
Blister cracks, like other cracks in a paint film, invite moisture penetration and later separation of the paint from the coated surface. Moisture behind the paint layer leads to early paint failure. Lots of experts know that. But an accurate diagnosis of the precise paint failure mechanism distinguishes between building interior moisture, building leaks, and an external failure of the paint coating due to having been applied in hot sun or over a too-wet primer coat.
Depending on the chemistry and intended application of a particular paint, if applied in windy conditions, the paint may form a skim coat too rapidly, retarding evaporation and leading to cracking or blistering failure as just described above for the "painting in sun" case.
Be sure to follow the recommendations of your paint supplier and paint manufacturer in choosing the right paint for the proper application.
Painting in sun or wind can be a particular problem when painting new stucco surfaces. See PAINT on STUCCO, FAILURES for details.
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