Causes of Paint Failures on Stucco Exterior Walls InspectAPedia® -
Stucco wall paint failures
Stucco exterior cracks, white deposits, stains, efflorescence causes
Causes of white efflorescence bloom on stucco building walls
What is saponification and how does it cause paint adhesion and cracking failures?
How do stucco pH and moisture impact the success of a stucco paint job?
How to diagnose the cause of failing paint on a building
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This article reviews 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.
STUCCO WALL PAINT FAILURE INDICATORS - Paint Failure Indicators Help Diagnose Paint Problems on Stucco
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
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
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.
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.
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.
Tips for Avoiding Paint Failure on New Stucco Exteriors
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
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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KTA-Tator, Inc.,
Corporate Headquarters,
115 Technology Drive,
Pittsburgh, PA 15275,
: 412.788.1300
Fax: 412.788.1306
E-mail: info@kta.com KTA is a Consulting/Engineering firm founded in 1949 in Pittsburgh, Pennsylvania. Their main specialty is consulting and inspection in the protective coatings and construction industries.
Painting Contractor Newsletter on Alkalinity and Efflorescence in New Stucco, The Paint Quality Institute, in which the authors point out that high pH and efflorescence correlate. This article suggests that the painter should use a water-based primer/sealer, (Scott Paint recommends an acrylic stucco primer) then use an alkaline resistant 100% acrylic latex top coat, avoid bright colors, and apply two top coats to be sure that cracks and pinholes are sealed. The Paint Quality Institute is at PO Box 904, Spring House PA 19477. www.paintquality.com
Paint and Surface Coatings, Theory and Practice, R. Lambourne & T.A. Strivens, Ed., Woodhead Publishing Ltd., William Andrew Publishing, 1999 ISBN 1-85573-348 X & 1-884207-73-1 [This is perhaps the leading reference on modern paints and coatings, but is a difficult text to obtain, and is a bit short on field investigation methods - DF]
Analysis of Modern Paints, Thomas J.S. Learner, Research in Conservation, 2004 ISBN 0-89236-779-2 [Chemistry of modern paints, overview of analytical methods, pyrolysis-gas chromatography signatures of basic modern paints and their constituents, Fourier transform infrared spectroscopy for paint analysis, direct temperature-resolved mass spectrometry, and analysis in practice - technical reference useful for forensic paint science, focused on art works -DF]
Seeing Through Paintings, Physical Examination in Art Historical Studies, Andrea Kirsh, Rustin S. Levenson, Materials in Fine Arts, 2000 ISBN 99-051835 [ forensic science, technical reference, focused on art works - DF]
Paint Handbook: testing, selection, application, troubleshooting, surface preparation, etc., Guy E. Weismantel, Ed., McGraw Hill Book Company, 1981 [Excellent but a bit obsolete paint theory and practice, also a bit light on field investigation methods, out of print, available used-DF]
Art, Biology, and Conservation: Biodeterioration in Works of Art, RObert J. Koestler et als. Eds., Metropolitan Museum of Art, 2003, ISBN 1-58839-107-8
Building Pathology, Deterioration, Diagnostics, and Intervention, Samuel Y. Harris, P.E., AIA, Esq., ISBN 0-471-33172-4, John Wiley & Sons, 2001 [General building science-DF]
Understanding Ventilation, John Bower, The Healthy House Institute, ISBN 0-9637156-5-8, 1995 [General building science-DF]
Dampness in Buildings, Diagnosis, Treatment, Instruments, T.A. Oxley & E.G. Gobert, ISBN 0-408-01463-6, Butterworths, 1983-1987 [General building science-DF]
"Moisture Control in Buildings: Putting Building Science in Green Building," Alex Wilson, Environmental Building News, Vol. 12. No. 5. [Good tutorial, "Moisture 101" outlining the physics of moisture movement in buildings and a good but incomplete list of general suggestions for moisture control - inadequate attention given to exterior conditions such as roof and surface drainage defects which are among the most-common sources of building moisture and water entry.--DJF]
"Staining and Microbiological Infestation of Acrylic Paintings on Hardboard", Ulrik Runeberg, Conservator (Dipl. Rest./M.A.), Museo de Arte Contemporáneo de Puerto Rico, San Juan Presented,April 2007 conference in Richmond Virginia, sponsored by the AIC (American Institute for Conservation), this paper discussed the staining and microbial infestation of acrylic paintings on hardboard. - private correspondence, ER <->DF 12 September 2006. The following quotation is from the paper's abstract:
"Hardboard served as a common and popular support for many modern paintings that were carried out from the mid - 1920’s, and still is used occasionally in contemporary art. Many artists rejected hardboard as an inferior industrial construction material of low aesthetical value, whereas others considered the processed and compressed wood fiber boards to be a stable, light and economic alternative to solid wood panels and other rigid supports.
"From the conservator’s critical point of view, the many disadvantages of this type of support include: high acidity, hygroscope characteristics, tendency of ‘off-gassing’, (>tendency of) warping, occasional flaking of painting material in the case of tempered hardboard. The deterioration of paintings on hardboard depends on a number of factors including: the quality of the hardboard, prevailing storage conditions, and the preparation of the support by the artist. While there are many paintings on hardboard that are in very good condition, this paper will focus on those paintings that are heavily deteriorated and damaged.
"A very characteristic damage found on porous painting layers such as acrylic colour on hardboard, is the formation of stains. Generally, those stains are described without any differentiation as ‘fox-spots’. The examination of various paintings concerned led to the conclusion, that there exist different kinds of stains that need to be discriminated against each other, to ensure an appropriate conservation and restoration treatment.
"This paper aims to characterize and differentiate the stains, and will provide preventive and practical treatment proposals for the conservation and restoration of affected paintings. Questions such as ‘What are the stains composed of?’ and ‘Which may be the causes?’ will be addressed. Stains may consist of a variety of contents, such as: Ligneous residues, fungal infestation, bacterial activity, a combination of microbial and support induced discolouration [SID], a ‘symbiotic relation’ of ‘SID’ and fungal infestation, or the blooming of ingredients from the original painting materials. A range of microscopic analysis of the actual microbiological infestation of selected samples will be provided. The paintings that were examined, sampled and treated, are part of the Puerto Rican heritage, and were all kept in excessive humid tropical conditions, before they entered the Conservation Department of the Museum of Contemporary Art in Puerto Rico.
"Conservation treatment options of stained paintings on hardboard will be discussed. A high level of acidity (caused from SID and/or micro-organisms) may require measures of reduction, disinfection and neutralization. Treatment methods that reduce the ligneous stains and residues of micro-organisms, and neutralize affected areas in painting layers include stain removal through the application of soaking compresses (poultices), and de-acidification through alkaline material.
"Other aspects of deterioration, that do not have to do directly with the formation of stains, but also are typical for hardboard as painting support, will be mentioned briefly." - U.R.
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