Stucco wall paint in San Miguel de Allende Mexico (C) Daniel FriedmanCauses of Paint Failures on Stucco Exterior Walls

  • SIDING EIFS STUCCO PAINT FAILURES - CONTENTS: Stucco wall paint failures: cause, diagnosis, cure & prevention. Stucco Paint Failure Indicators Stucco exterior cracks, white deposits, stains, efflorescence causes - Causes of white efflorescence bloom on stucco building walls. Stucco pH & Stucco Paint Failure How do stucco pH and moisture impact the success of a stucco paint job? Water & Weather & Painted Stucco. Avoiding Paint Failure on New Stucco What is saponification and how does it cause paint adhesion and cracking failures? Correcting Paint Failures When Re-Painting
  • POST a QUESTION or READ FAQs about diagnosing paint problems on exterior or interior stucco building surfaces

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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.

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STUCCO WALL PAINT FAILURE INDICATORS - These Paint Failure Indicators Help Diagnose Paint Problems on Stucco

Painted stucco San Miguel de Allende Mexico (C) Daniel FriedmanThe 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.

[Click to enlarge any image]

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.

Her 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.

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

  • Painting over un-sound stucco or on even a brand new stucco wall that has not been properly prepared and cured. See our photo of antique stucco on an adobe home wall just above.
  • Stucco wall cracking that maps most color changes and efflorescence found together on a building often indicate insufficient hydration during stucco curing and failure to seal the cracks with an elastomeric stucco patch compound prior to painting.
  • Failure to sufficiently hydrate the stucco during curing, especially in hot windy climates. Hydration, wetting down the stucco extracts the high levels of lime that are often present to provide workability of the stucco coating during application on the building.
  • Rushing the stucco paint job: because the contractor may want to use the scaffolding that was set up to apply the stucco, the painting contractor may be tempted to paint before the stucco was adequately cured. Painting over stucco that is inadequately cured (high pH) or still wet (high moisture) can lead to a saponification failure, especially at locations where subsequent leaks or water wet the stucco substrate. See Stucco Paint Saponification Adhesion Failures below.
  • Surface conditions of the stucco at time of painting - cure of the stucco, pH, cracks, moisture level, temperature, wind, sun exposure, even dirt or dust
  • Method of application of paint coatings on stucco, spray or roll-on of the paint for primer and top coat, including also the number of coats applied.
  • Paint selection for use on stucco: compatibility between primer and top coat, compatibility with new stucco, whether a primer was really applied, whether the paint was thinned excessively or was otherwise defective or even intended for this application. The paint product(s) that have been used should have been selected and applied according to their manufacturer-recommended application methods
  • Paint company and paint industry standards for painting stucco and recommendations for painting over stucco need to be respected
  • Stucco wall moisture level and stucco wall pH measurements made by the painter during the painting process - were they made at all and were they made properly?
  • History of the stucco surface paint job, the dates of stucco application, date of painting, weather conditions may indicate a rush job or special rain, sun, wind, or freezing exposure weather problems that ruined the paint job before it had a chance to dry and cure.
  • Comments by other onsite investigators and their reports, if any, can provide important diagnostic details
  • Proposals for remediation/repair re-painting work on a stucco paint failure necessary at the property, if any, can also provide important diagnostic details

Stucco Paint Saponification Adhesion Failures

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:

Saponification not only affects paint adhesion. the acrylic film becomes rigid, possibly leading to hairline cracks in the stucco coating. Exposure to the alkaline solution from the stucco forms a hydrophilic [water resistant] layer of low molecular weight calcium soap beneath the [paint] film that attracts additional water and causes the saponification to spread.

Ultimately, the chemical "anchor" [holding the paint onto the stucco surface] is removed from the film which results in diminished adhesion. A paint saponification failure mechanism can be further supported by the location of the failures on the building. Look for specific locations of paint adhesion failure, contrasted with a more uniform paint adhesion failure over all of the structure, on all sides and locations.

Disbonding [breaking the paint-to-surface bond] is generally observed only in areas exposed to water permeation through unscheduled openings in the structure [such as leaks at poorly-sealed trim or at penetrations added for fixtures].

In a saponification failure of a painted stucco surface, most of the surface area sealed by the coating and not exposed to water will be found soundly adhered with no signs of deterioration.

Water permeation of the stucco substrate at ledges, mortar joints, edges of balconies, and areas near the ground which were wetted by capillary action, cause the alkaline salts in the stucco to be leeched into the water, resulting in a stucco paint saponification failure. - KTA Tator, referred to InspectAPedia by conservator Ulrik Runeberg (See PAINT ANALYSIS USES). Definitions of esther and carboxylic acid - Princeton University.

Stucco pH as Contributor to Paint Failure

Photograph of  peeling paint on a building exterior - can you diagnose this failure by eye?When painting on stucco, the pH (alkalinity) of the surface as well as moisture trapped under paint have been associated with efflorescence or white blooming problems.

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 & Weather Impact on Inspection of Painted Stucco Surfaces

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.

Stucco before wetting (C) Daniel Friedman Stucco before wetting (C) Daniel Friedman

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.

Stucco wall stains after painting (C) Daniel Friedman Stucco wall stains after painting (C) Daniel Friedman

As detailed at STUCCO WALL METHODS & INSTALLATION and also in the printed text Best Practices Guide to Residential Construction:

The Importance of Weather to Stucco Work and Stucco Paint Coatings

See details about the cause, diagnosis, cure, or prevention of paint failures on stucco exterior walls, found at STUCCO PAINT FAILURES.

EIFS Failure due to poor installation practices (C) Daniel FriedmanMoisture, 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.

Stucco effloresence (C) Daniel FriedmanDirect 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.

Tips for Avoiding Paint Failure on New Stucco Exteriors

Stucco wall stains after painting (C) Daniel FriedmanOur 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.

  • Hydrate the stucco: Be sure that the stucco is adequately hydrated during curing. Washing down the stucco in hot dry climates and keeping it hydrated avoids cracking and the development of white efflorescence blooms on the building surface.

    Typically in hot or windy conditions the new stucco is hydrated by the stucco contractor for two or three days to slow the stucco curing rate and to permit the chemistry of stucco curing to complete to a sufficiently hard and impermeable surface.
  • Be sure that the stucco has adequately cured before painting. Typically the stucco must cure for at least ten days to two weeks before it can be painted. The pH of cured stucco is normally below 11. If the stucco is still "hot" - not cured - painting it risks early paint failure.
  • Fill cracks and holes found in new stucco using an elastomeric stucco patch compound before painting, and be sure that the patches have also cured according to the manufacturer's recommendations.
  • Make proper moisture & pH measurements of the stucco before painting. High pH (over 11) stucco is likely to give a problem for the new paint job.

    When measuring pH on a wall, don't just measure the obviously "ok" areas, measure where you see fine cracking, early signs of efflorescence, areas of paint failure in a primer coat, and in representative sun-exposed and shady wall areas. Different exposures of a building's stucco exterior may cure at different rates because of variations in sunlight and wind. Follow the pH and moisture test and level recommendations of the paint manufacturer.
  • Use a water based masonry primer or sealer as the first paint coat. This helps avoid trapping moisture in the new stucco wall. Be sure that the paint is one that the manufacturer recommends for this application, and that it is applied at the thickness (rate of coverage) recommended by the manufacturer. Don't over-thin the paint in the can nor on the surface.
  • Be sure that the primer coat is thoroughly dry before applying a second coat.
  • Roll-in or brush paint coatings on difficult building surfaces. Relying on spray-alone on building surfaces may do a great job of getting the paint onto the surface quickly (economically) but on some surfaces the paint needs to be worked into the surface with a brush or roller to coat across hairline cracks or to bond well.
  • Top coat paint on stucco should be a high quality 100% acrylic latex paint that resists alkaline surface chemistry and the formation of efflorescent blooms or white blotches. Some paint manufacturers recommend two top coats as well as one primer coat on new stucco exteriors. Some paint failures we have investigated were due in part to poor quality or choice of incompatible paints themselves. Don't use bright colors that are non-durable and that will fade quickly in strong sun exposure.
  • Apply the paint in thickness recommended by the manufacturer. Use a wet paint gauge if necessary to obtain the proper coverage. One of our assistants at a paint job tried to save money by too-thin application of the paint - a step that cost us extra because we had to paint the entire surface an extra time. Adequately thick coating of paint combined with proper working of the paint onto and into the surface will seal pinholes and hairline cracks in the stucco coating, thus avoiding wind-blown rain leaks into the wall and early paint failures.

A Typical Proposal for Correcting & Re-Painting an Improperly Painted New Stucco Surface Includes

  • Trenching around the building walls to permit painting down to ground level
  • Pressure-washing the building exterior surface, followed by adequate dryout time
  • Patch and seal all stucco cracks using a recommended elastomeric patch compound; caulk at openings and penetrations where the caulk is not in sound condition
  • Spray and back-roll or brush-on a water based or more likely in a retrofit repair job, a 100% acrylic primer recommended for use on stucco walls. For example, Dunn-Edwards Paints recommends specific primers that should be applied after cleaning off any white residues from efflorescence or similar stains. Quoting from the company's advice for applying one of their Acrylic exterior paints:

    All surfaces must be cured, clean, dry, and free from dirt, dust, rust, stains, grease, oil, mildew, wax, efflorescence and other contaminants. Remove all loose, peeling, or chalky paint by sanding, scraping, high-pressure washing or other appropriate methods. Repair all cracks, holes, and other surface imperfections with a suitable patching material. Repaired surfaces should match the surrounding surface texture. If efflorescence exists, remove all noticeable deposits and prime the entire surface with Super-Loc® (W 718), Eff-Stop® (W 709) or Acri-Loc® (W 6232).

  • Spray and if necessary roll or brush in a top coat of 100% acrylic satin paint recommended for use on stucco walls
  • Inspect each painted layer for compliance with the manufacturer's recommended wet-thickness during application
  • A warranty on the finished surface (5 years on paint, 20 years on stucco are common)


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If your stucco on foam siding extends below ground, also see Insects & Foam Insulation

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PAINT on STUCCO, FAILURE at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

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