Gray Mottling Plaster Discoloration #5
This fifth article provides additional evidence that gray (or grey) mottling discoloration (which often takes several months to develop) is caused by improper plaster workmanship.
In 2003, the National Plasterers Council (NPC) commissioned the National Pool Industry Research Center (NPIRC) at Cal Poly in San Luis Obispo to conduct research experiments on plaster degradation, discoloration, and other durability issues. The second phase of their studies (2004-2005) consisted of twelve pools being plastered with one-half of each pool troweled with a small amount of water added to aid in lubrication of trowels and with the other half troweled without any water added.
The reason for this comparison was because the NPC wanted to prove that adding a little water to lubricate the trowel didn’t cause “etching” (although no one has claimed that it does) and that adding water during finishing was okay to do, even though the cement flatwork industry strongly discourages it.
While the study didn’t reveal significant differences between the two sides on the issue of etching, interestingly, it was observed in the balanced pools that the “no water added” side had significantly more discoloration than the “water added” side. Also, Pool 8, the aggressive pool, did not develop as much discoloration in comparison to most of the balanced pools. These inspections were performed several times over the course of eight months.
Despite the known fact that troweling when cement and plaster is somewhat hard or dry can lead to discoloration, the Cal Poly professors did not make this a topic of discussion in the “Conclusions and Recommendations” section of their report.
Instead, their report (incorrectly) concludes that “wet troweling” is okay to do, and suggested (without proper scientific evidence) that most plaster problems, (including discoloration, deterioration, and craze cracking), were caused by aggressive water chemistry, salt pools, borates, or (supposedly) inaccurate test kits.
Also conspicuously missing from the NPIRC study was an inspection to determine the amount of discoloration on the plaster coupons with high contents of calcium chloride added (2% to 5%), which is also known to cause graying of cement. Once again, the NPIRC avoided implicating defective plaster or workmanship. Does anyone believe that something is not amiss with this research?
Consider this; some plasterers occasionally use high amounts of calcium chloride to accelerate the hardening of pool plaster in order to finish their work quicker. When doing so, the plaster mix may sometimes harden so fast that finishers end up performing late, hard, and dry troweling, and perhaps add excessive amounts of water to compensate and get the plaster surface smooth. According to research by independent cement laboratories and the American Concrete Institute, that is a recipe for causing darkened or gray mottling discoloration, which sometimes also develops white “soft” spotting or streaking along with the graying.
Since the completion of the study in 2005, there has not been an admission by the NPC regarding the incomplete or misinterpreted data involved in the above results from their study. As an industry association, the NPC should acknowledge that graying and streaking can be caused by improper workmanship, and should not be suggesting that the NPIRC has proven that aggressive water chemistry or improper chemical start-ups cause this discoloration.
The NPC should be a leader and adopt standards that will promote quality plaster work that will improve the durability and cosmetic appearance of the popular and economical white plaster pools. They should be educating their members on proper workmanship techniques. They should stop suggesting that white plaster is inherently weak and can’t withstand “imperfect” water balance chemistry. Yes, Quartz and Pebble pools can be more durable, but they are more expensive and can also experience discoloration and other similar defects from poor workmanship and/or materials.
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