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

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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 discuss the following: Stucco Paint Failure Indicator,s 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.



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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 Friedman

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.

[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

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 PAINTING on 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.

Watch out: 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

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.

Stucco before wetting (C) Daniel Friedman

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

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

Stucco wall stains after painting (C) Daniel Friedman

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 Friedman

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 above 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.

White blooms and stains in stucco paint jobs may be due to painting when the surfaces were too dampe or painting when the paint would be exposed to cool or cold temperatures or dampness, dew, rain, before the paint was cured.

See PAINT FAILURE SURFACTANT LEACHING for details.

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 also PAINTING in SUN or WIND.

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 Friedman

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.

Tips for Avoiding Paint Failure on New Stucco Exteriors

Stucco wall stains after painting (C) Daniel Friedman

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.

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

Use of Bonding Agents in Stucco Applications

We have moved this dicsussion to a new article: see STUCCO & CONCRETE BONDING AGENTS

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Continue reading at PAINTING SHORTCUT ERRORS or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see STUCCO & CONCRETE BONDING AGENTS - use of bonding agents with stucco and concrete coatings & surfaces

Or see PAINT on STUCCO FAILURE FAQs - questions & answers posted originally at this article

Or see STUCCO OVER FOAM INSULATION

If your stucco on foam siding extends below ground, also see INSECTS & FOAM INSULATION

Or see PAINT FALURE, DIAGNOSIS, CURE, PREVENTION - home

Or see PAINT FAILURE DICTIONARY for terms & examples of all types of paint failures, stains, cracks, gripes.

Or see these

Paint Failure Indicator Articles

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