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Paint failures on stucco walls, cause, diagnosis, cure & prevention methods: this article describes common building exterior & interior painting mistakes when painting on modern stucco building exteriors, 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. The diagnosis and cure of paint failure on buildings, particularly on wood siding and trim, is quite possible if there is a careful and thorough inspection of the building, its history, its surfaces, and the actual points of paint failure. It is diagnostic to compare the same coating on the same type of surface at different locations on a building and in areas of failed and not-failed paint.
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STUCCO WALL PAINT FAILURE INDICATORS - These Paint Failure Indicators Help Diagnose Paint Problems on Stucco
Related articles: 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. Also see STUCCO OVER FOAM INSULATION, see SIDING EIFS & STUCCO and review POLYISOCYANURATE FOAM INSULATION. If your stucco on foam siding extends below ground, also see Insects & Foam Insulation
The combination of cost and schedule pressures and lack of information about the cause of stucco paint job failures may lead some painting contractors into trouble, especially at new construction sites.
The combination of high lime stucco, schedule and cost pressures, and failure to appreciate the importance of stucco hydration and curing prior to painting lead to stucco cracks, white blooms of efflorescence salts in some areas of the stuccoed surface, and early paint job failure - sometimes in less than a year after painting.
Typical field investigation of paint failures on stucco exteriors involves recording the pattern and extent of paint failure on all building surfaces, possibly correlating paint failures to different building sun or weather exposures, measuring the moisture content of the stucco (of course this may not directly indicate what the moisture content was at the time of paint application), chemical testing of paint and stucco samples, and knife probing or parallel razor cuts and tape testing to assess the adhesion characteristics of the painted coating.
Typical causes of or contributors to paint failure on stucco exteriors and other information that we consider when diagnosing paint failure on stucco include the following paint failure causes and signs
Saponification of the paint binder - adhesion loss on painted stucco: saponification at the contact point of paint on a stucco surface: saponification refers to a process also called alkaline hydrolysis: water and high alkalinity [see "Rushing the stucco paint job", above] breaks an ester [a class of organic compounds that react with water to produce an alcohol and an acid] down to a carboxylic acid [an organic acid -COOH or -CO2H, typically a weak acid] and an alcohol. If the pH of the stucco surface continues at an alkaline level, which often happens when raw stucco is painted-over too soon, carboxylic acid will be be detected as carboxylic acid salt - (a carboxylate anion with metal cation, such as Na or Ca. Saponification weakens the paint film adhesion at the surface of the stucco.
Water or wet stucco combined with high pH is what creates a saponification-adhesion-loss problem on stucco and can also cause hairline cracks in the stucco coating. Here is a more technically detailed explanation of the stucco saponification adhesion failure problem, with editing by DF:
Field test of stucco pH: A simple field test can measure the alkalinity of the stucco: A small sample of the stucco is removed from the building, powdered and added to an equal volume of distilled water. Do not use tap water. If the measured electrical resistance in the solution is low, and if the chloride concentration is high there is a considerable level of chloride-based electrolyte in the sample. Measure the pH of the sample. If it is high ( pH was 11) the stucco sample is very alkaline - a neutral pH is 6-7 range.
White powdery blotches appearing in the painted stucco surface are usually blooms of efflorescence caused by painting over cracks or other areas of extra moisture absorption in the stucco surface. Where recently-applied stucco was not adequately cured, and where surface alkalinity remained too high (pH over 11) white efflorescence blooms are particularly common. This painting error, sometimes the fault of rushing the paint job, leads to both cosmetic defects and early paint failure.
While a painter reports having taken some pH measurements with acceptable results, our field work has consistently found that both moisture and pH vary significantly over a building surface. When relying on measurements (and thus rushing the paint job schedule or painting “early”) rather than allowing more elapsed time in deciding when to paint a building, a common error is to rely on “safe” readings obtained in some areas while failing to measure or attend unacceptable moisture or pH level readings in other building areas.
If on a building the stucco was applied in very hot dry conditions (no surprise in Arizona, for example) and was if the stucco inadequately wet down (hydrated) during cure, that could also have left areas of high pH, making the pH measurements we cited above critical when deciding when to paint or whether additional surface preparation was needed.
See the stucco painting advice articles we cite at References below.
Efflorescence on building surfaces, including on a painted stucco surface is described at Efflorescence & white or brown deposits.
Water or wet stucco combined with high pH is the problem. It is the combination of painting over a still-wet stucco surface or still damp surface, or a surface that is subsequently exposed to abnormal wetting, along with high alkalinity that causes saponification of an acrylic paint on stucco. When painting a sufficiently dry stucco surface, alkalinity alone will not cause this problem. - paraphrased from KTA Tator, a Pittsburgh consulting firm.
As our photos just below demonstrate, the appearance of any painted surface, particularly new stucco, can be significantly different when it is wet.
While there is nothing abnormal or "wrong" with a painted surface that looks a bit different when wet, say darker in color, streaks or the appearance of mottled efflorescence or white blooms on a wall after wetting may be telltales of a paint problem, and certainly these inconsistencies mean that a paint failure investigator needs to inspect when the surface is dry. Inspecting in the rain or just after raining or other sources of wet on a building exterior may lead to incorrect conclusions.
White Run-Down Stains on Exterior Stucco after New Paint Job
Below we show several photographs of ugly white stains that appeared quickly after a reader's home's stucco exterior was spray painted in 2010. The reader indicated that the painters applied a Dunn-Edwards exterior flat acrylic paint very quickly, perhaps too quickly, after the home had been power-washed. The stains are most likely not due to a defect in the paint itself (unless it was amended or over-thinned by the painter) and more likely due to improper surface preparation combined with painting before the surface was dry after power washing.
See details about the cause, diagnosis, cure, or prevention of paint failures on stucco exterior walls, found at STUCCO PAINT FAILURES.
Moisture, humidity, rain, or wet conditions during thin-coat or EIFS stucco work can lead to a subsequent series of failures of the entire installation. The home shown in our photo (left) was the subject of litigation. We observed that the final stucco had been applied over wet surfaces and in some cases over surfaces that also had been troubled by soil that had splashed-up on the building during rainy weather.
Stucco wall paint failures are also traced to moisture, efflorescence, and failure to adequately clean the exterior and then allow it to dry before painting. See P
Temperature during stucco work will speed up or slow down the hydration process that cures the cement in stucco. It is best to avoid application in extremely hot or cold temperatures. In hot, dry, and windy weather, frequent misting will be required on the scratch coat or the installer may need to tape polyethylene sheeting in place for proper curing.
Direct sun tends to dry out the fresh stucco too fast, so installers should try to follow the shade around the building. Also, retardants are available that can be sprayed on the scratch or brown coat in hot weather to slow down the curing.
Sun, heat, and rapid drying conditions can present special stucco application troubles or subsequent stucco paint coating troubles in hot dry climates such as the American Southwest. (Photo at left).
Cold weather also presents problems. Stucco should not be applied under 40°F, and it should not be allowed to freeze within 24 hours of application. Accelerators can be added to the stucco mix in cold weather, but these can weaken the material, and calcium-based accelerators can lead to efflorescence.
Heating the materials and, if necessary, tenting the structure can permit work to proceed in cold, even freezing, weather.
Cool, moist weather is ideal for traditional stucco wall installations. In humid weather, with relative humidity over 70% or heavy fog, misting is not usually required.
Our best guess is that the run-stains down this newly painted stucco wall (photo at left) as well as the stains above are consistent with wet areas in the stucco and uneven drying following power washing.
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