Specifiers: Mystery of the Yellow Stains

A newly constructed, twelve-floor high-rise building developed an unusual staining problem on the interior ceilings. The building has re-enforced concrete floors that formed the ceilings of the floor below. Metal re-enforcing bar imbedded into each concrete floor protruded from the ceilings, was cut and ground smooth, then coated by brush with an alkyd zinc-chromate, anti-corrosive primer. The specification had called for a water-based, gypsum plaster texturing compound to be applied to the entire ceiling surface.

The texture application was started in the early mornings, and continued until late evenings in order to "keep on schedule". A few days after the first floors were completed, painters returning to prepare to paint the wall surfaces noticed yellow stains on some areas of the ceiling. The texture applicators removed a few of the small areas where the staining occurred and determined that it only appeared where the re-enforcing bar had been protruding. Believing that the painters hadnĂ­t completely coated the steel, and that the stain was caused by rust, the texture applicators removed all of the stained spots and re-primed the areas with more zinc chromate primer, then repaired the texture.

The stains re-occurred as before. The Association was asked to send out a certified coating inspector to examine the problem. Samples of the texture were taken to a laboratory for analysis that revealed that the stain was actually zinc chromate. Fresh samples of the texturing compound were then prepared in the lab and measured for pH. The results showed that the texture had a highly alkaline pH of 13.3. Water content of the recommended mix was approximately 62 percent by volume.

In further lab work, it was found that the zinc chromate pigment in the primer was partially soluble in alkaline water at a pH of 11 or greater. This, with the high water content of the texturing mix and slow curing (when applied late in the afternoon or during times of high humidity) allowed the stains to leach out to the surface of the texture.

Since the water ratio and pH of the texture compound was critical to its working properties, appearance, and curing, the stained areas would have to be treated with a sealant. A fast-drying, white, phenolic alkyd primer was selected. This had the added benefit of stopping the staining but did not require removal of the texture. The balance of the ceilings not yet textured had all the zinc chromate primed spots treated with the white alkyd. No more staining occurred!

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