KTA-TATOR,
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Phone: 412.788.1300
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The Case of the Reappearing Staining
By Rick Huntley, Coatings Consultant, KTA-Tator, Inc.
A few years ago a reputable contractor constructed several single family homes in various housing developments in a southeastern state. The interior walls were of dry wall construction and were coated with a flat latex interior wall paint manufactured by a well known paint company. overall, the houses were of quality construction.
Several months after completion of the first few units, residents complained that stains were evident on the interior walls. The dark staining appeared in straight lines, both vertical and horizontal, and in certain cases could be seen through the wallpaper.
The contractor immediately inspected the homes and confirmed the existence of the staining. Further, he noted the stains were limited to the drywall joints and to the presence of joint compound. The contractor sought the advise of the paint manufacturer and joint compound manufacturer as the problem appeared to be widespread.
The coating manufacturer's representative evaluated the problem in the field and concluded that it was a mildewing phenomena, as the homes were located in a hot and humid climate. Confident that their coating was not at fault, they submitted samples of the joint compound to their laboratory to determine if proper levels of mildewcide were present. It was determined that the joint compound contained a sufficient quantity of mercury, a common element in mildewcides. Thus it was unlikely that mildew was actually growing in the joint compound. After further testing, they concluded that the staining was caused by some form of a heavy metal material leaching from the joint compound.
At the same time, the joint compound manufacturer hired an independent laboratory to investigate the problem. The laboratory made several preliminary observations including the following:
- The staining phenomena disappeared when household bleach was applied.
- Only one coat of paint was applied over the tape joints; no primer
(sealer) was evident.
The laboratory investigation revealed that mildew spores were present in the paint.
Accordingly, the following failure mechanism was presented by the joint compound manufacturer: condensed water on the surface was being drawn through the pores of the paint and into the unsealed joint compound. This "retained" moisture was causing mildew to grow on the topcoat.
Upon receiving a copy of the laboratory report, the coating manufacturer sent technical representatives to closely inspect the stained walls. They ran several field chemical spot tests on the stains attempting to determine their origin. They confirmed that the bleach removed all of the stains. Further, oxalic acid, phosphoric acid, and ammonium sulfide had no affect on the staining. With the exception of visible stains observed on areas of the ceiling where no paint had been applied, field observations were similar to those of the joint compound manufacturer.
Several paint chips were removed from the walls and returned to the laboratory. Upon microscopic examination, the presence of mildew colonies or spores were not found. The chips were then forwarded to a microbiology laboratory where they were placed in an agar atmosphere (a standard test to determine what species of living organisms are present in a material) for three to five weeks. The test revealed that no living mold or mildew was present in the paint chips.
The contractor was then faced with a distressing situation. He had received two conflicting reports claiming different causes for the staining phenomena. Dissatisfied with the results to date, the contractor decided to take matters into his own hands. On several of the units he applied a solution of household bleach which appeared to remove the staining. In other units, he applied another coat of the original interior flat latex paint to hide the stain. For nearly a month the contractor thought he had remedied the problem.
However, the remedy was short lived. After approximately one month, angry residents began complaining that the dark stains were gradually reappearing on areas where a second coat of the latex paint had been applied. Dark stains were rapidly reappearing where the bleach had been applied. At this point, the contractor contacted KTA-Tator, Inc. to independently evaluate the cause of the staining and to recommend a permanent remedy.
One of the residents allowed the contractor to remove a 6" x 6" section of dry wall with a joint running down the middle. The sample contained tape and joint compound, and the interior flat latex paint. A dark stain was clearly evident over the area where the joint compound had been applied.
Upon receipt at the KTA laboratory, the sample was first viewed microscopically, common in an investigative analysis. The entire surface of the sample showed a moderate amount of pinholing, however no significant difference in the degree of pinholing on the painted portion of the joint compound versus the -3- paint applied directly over the drywall was evident. Further, no evidence of mildew on the top surface of the paint, or in the joint compound was found. Under magnification, mildew is easily recognized by its tendrils and spores.
The KTA laboratory then treated select areas on the sample with different chemical solutions (spot tests) in an attempt to determine the nature of the staining.
The first area was a one inch wide strip running the length of the sample, perpendicular to the stain. Household bleach was applied to this area, which immediately faded the dark stain. bleach had no affect on the unstained area.
A similar one inch wide strip was treated with a three percent solution of hydrogen peroxide. The solution had no affect on the stained or unstained areas.
The next area, located on the unstained paint, was treated with a one percent solution of sodium sulfide. The solution had no visible affect on the unstained area. However, the same one percent solution of sodium sulfide applied to the stained portion of the paint that had previously been treated with the bleach solution left a dark stain.
The paint was then removed from a small area over the joint compound. In this area, the one percent solution of sodium sulfide was applied, and a dark stain appeared. Household bleach was then applied to the stain, and it immediately faded.
The laboratory spot testing clearly indicated that the cause of the discoloration along the joint was sulfide staining of heavy metals in the joint compound. Sulfide staining is common in coatings, but is more frequently associated with lead-based paints. In this case the sulfide was reacting with the mercury fungicide in the joint compound.
The sulfide staining on the interior walls of the houses was caused by a combination of two factors: extreme southeastern humidity, and the presence of high levels of hydrogen sulfide in the air caused by local mining operations. The humid air permeated the porous latex paint and produced water. When the water evaporated from the joint compound, it carried some of the heavy metal to the surface of the latex paint. While at the surface, the heavy metal reacted with the hydrogen sulfide atmosphere, thus causing the dark staining.
Laboratory testing confirmed that the household bleach immediately faded the stain, giving the false impression that the staining was caused by mildew attack. Sodium hypochlorite, the active ingredient in bleach, is often used to differentiate between dirt and mildew stains. It is theorized that, in this case, the bleach reacted with the black mercury sulfide compound, thereby changing its color. The rate of the staining was a function of the rate of heavy metal leaching to the surface. This explained why, when ammonium sulfite was applied to the surface of the stained area, no change was observed. All the mercury available for reaction at that site had already reacted.
Household bleach faded the staining, but the remedy was only temporary. Soon after application, the bleach volatilizes leaving no permanent protection; reaction of the mercury with the hydrogen sulfide reoccurs.
Even though the failure mechanism was determined, the method of how to remedy the problem remained. As it was unlikely that the new homeowners would agree to bleach their walls every month or so, it was obvious that a more permanent solution was necessary.
KTA suggested that either an alkyd wall paint or an acrylic emulsion stain blocker be applied to the interior walls. Both coatings have a greater resistance to moisture permeability when compared with the less expensive flat latex interior wall paints. Either product should effectively cover the existing staining and prevent its reoccurance.
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