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Sometimes, a gray and darkened discoloration will develop in new white plaster swimming pools. This type of graying, where the plaster is very smooth, can occur in various patterns.

What is the cause of this unfortunate and occasional problem with plaster swimming pools?  Can the problem be avoided or fixed?  This problem is controversial, and culpability is being contested by pool plasterers and the pool service industry.

I am currently part of the pool service industry, but my father owned a pool plastering company where I worked for many years.  I bring years of experience in both industries, as well as years studying and experimenting pool plaster issues using empirical data from controlled studies.  My motivation is to elevate the pool industry to needed standards that will improve the product for homeowners, as well as pool builders and service techs.

The research conducted by on Balance, as well as other independent studies, show that new white pool plaster/cement can be discolored by improper workmanship practices, such as adding too much calcium chloride (a hardening accelerator) to the plaster mix, and mainly by overly late hard troweling. It is also known that the gray color becomes more noticeable a few months after the new pool is filled with water.

Research by ACI and PCA has documented discoloration problems with improper concrete flat work. The cause/effect is virtually the same as with pool plaster because both plaster and concrete contain a significant amount of Portland cement. (Concrete Slab Surface Defects: Causes, Prevention, Repair – Portland Cement Association 2001).

In the picture to the side, note the left side of the cement sidewalk is not discolored, but the right side is a darker gray color.  This an example of how improper late hard troweling can result in a dark discoloration.

Some plaster people believe that aggressive tap water causes gray mottling discoloration. However, note the above concrete sidewalk has undoubtedly been subjected to rain (which is very aggressive water), and which did not cause a discoloration of the properly troweled left side. Obviously, the concrete industry understands what the issues are, and what is not.

Also, there is general agreement that “acid startups” don’t result in gray mottling. Therefore, smooth graying discolorations (omitting possible scaling or metal staining issues) have nothing to do with improper start-ups or water balancing.

Various methods have been used to mitigate or eliminate grey mottling. Some plasterers will perform an acid treatment. But remember, acid treatments often etch and cause roughness of the plaster surface prematurely and may not remove the discoloration.

Some plaster consultants recommend a “torching” (heating) process to remove the gray color. However, cement authorities have determined that heating cement/plaster to very high temperatures detrimentally affects the long-term durability by causing micro-cracking and increased porosity. When heated to very hot temperatures, pool plaster will lose years of its’ workable life-span and will likely stain easier and faster afterwards.

Such harsh treatments are not what is best for the plaster surface. It is also not right to blame innocent service techs that did nothing wrong with water balancing to cause this plaster defect.

Therefore, considering the existing evidence and science, including the Cal Poly/NPIRC/NPC studies that also demonstrated that gray discolorations did not develop in their aggressive water test pools, the NPC should publicly acknowledge that gray mottling or blotchy discolorations is a plastering defect problem to help avoid confusion and disagreements at poolside with plasterers, pool owners, builders, and service techs.

See this link for a demonstration of how white plaster turns gray: http://www.poolhelp.com/home/onbalance-research/onbalance-research/how-white-pool-plaster-becomes-mottled-and-gray-discolored

Kim Skinner