It’s an all-too-familiar story: The pool’s surface looked pristine when filling began — but a few weeks or months later, patches of small white spots have appeared along the walls or floor.

The cause of these marks varies depending on who you ask — and, in fact, a decades-long debate continues to rage about the origin of pool spots. Some plasterers say they’re being blamed unfairly for damage resulting from aggressive water chemistry, while some service technicians allege that improper plaster workmanship is more often to blame.

The stakes aren’t small. Legal settlements over spotted plaster can soar past the seven-figure mark, and judgments in these cases often make or break the reputation of a pool plasterer or service tech.

It’s no surprise, then, that both sides call in the heaviest artillery they can find: Expert witnesses, persuasive lawyers, scientific citations and anything else that might provide an edge on the opposition.

Here’s a look at the ongoing debate.

The players

The most prominent group of pool plasterers is the National Plasterers Council , an organization whose activities range from industry education to standards-writing to testifying in court for and against plastering companies. The NPC publishes an industry-standard technical manual on plaster workmanship and care, and organizes scientific seminars and classes across the country. The group helped establish the National Pool Industry Research Center at California Polytechnic State University, San Luis Obispo — a laboratory dedicated exclusively to the study of pool and spa chemistry.

In the late 1990s, a group of people on the pool service side of the industry began to voice discontent with the NPC’s contention that aggressive water chemistry is a primary cause of plaster spots. They felt that stance placed an unfair amount of blame on service techs. Three of these dissenting individuals — Kim Skinner, Que Hales and Doug Latta — went on to form the onBalance group, an organization that performs and finances independent research into the causes of spots and other plaster problems. OnBalance members agree with the NPC that plaster defects and aggressive water can contribute to certain types of damage.

Opinions divide, however, when the discussion turns to the type of harm that water chemistry is capable of causing, and when it is — or is not — primarily to blame. 

Though most of onBalance’s experiments are performed on job sites or in home labs, the group has also worked closely with some petrochemical laboratories to analyze plaster core samples.

The game

NPC officials have long emphasized that improper plaster mixes, such as those containing too much water and/or accelerant (calcium chloride), often contribute to surface damage. Improper troweling can be a culprit as well, the group’s published reports say. However, the NPC also holds that testing fill water is crucial for proper plaster care. This is because “soft” water — that is, water containing low levels of dissolved minerals — has a tendency to adversely affect new plaster surfaces.

Since the mid-1990s, the NPC has conducted controlled studies on pools, demonstrating aggressive water’s tendency to etch plaster surfaces. They classify this type of damage as plaster spotting, which they term “etching deterioration.” Throughout the 2000s, the group has also funded studies at the National Pool Industry Research Center at California Polytechnic State University, San Luis Obispo. The results of these studies, the group argues, provide further support for the connection between aggressive water and etching deterioration.

The OnBalance group, on the other hand, maintains that plaster spotting is not actually etching at all, but rather deterioration caused by preexisting weaknesses in plaster. This is due to improper mix ratios and/or finishing. Water chemistry, onBalance says, is never the determining factor in creating plaster spotting.

OnBalance’s argument is based largely on several petrographic analyses commissioned in the late 1990s and early 2000s, from research laboratories such as R. J. Lee Group Inc. in Pittsburgh and the Portland Cement Association ’s Construction Technology Laboratories (also known as CTLGroup) in Skokie, Ill.

The studies reported that several factors contributed to a weak, porous and softened surface. This included adding excessive calcium chloride to the plaster mix, applying excessive water onto the plaster surface during troweling, and troweling later than recommended or with excessive pressure.

From these studies, onBalance draws a distinction between its view and the NPC’s.

Plaster spots, onBalance contends, are not etched by aggressive water, but rather dissolved out of areas of the plaster that deteriorate due to pre-existing weaknesses.

“We feel that using the words ‘etching’ and ‘deterioration’ together confuses the issue,” Kim Skinner says. “The only way we can describe the breakdown of poor plaster is by using the term ‘deterioration’ — whereas we use the term ‘etching’ to describe aggressive water action on cement compounds.”

A properly mixed and troweled plaster surface can be etched by aggressive water, but only uniformly across its surface. When the pattern of spotting is nonuniform, something other than water chemistry is at fault.

— Kim Skinner, co-founder, onBalance

In other words, though onBalance agrees with the NPC that aggressive water can sometimes etch plaster, the group says that properly mixed and troweled plaster will never develop localized patches of spotting deterioration surrounded by intact and dense plaster — even when exposed to aggressive water. The two phenomena, they contend, are reflective of two fundamentally different processes stemming from two entirely separate causes.

The referees

Professional chemists and engineers agree that spotting is indeed likelier to appear in some areas of the plaster than in others. However, some are less enthusiastic about the conclusion that the causes of spotting are so different from those behind other types of plaster damage — or that any individual culprit can be regarded as separate from the process as a whole.

“Regardless of what terminology you use, you’re still talking about a chemical being drawn out of a mixture,” says James Schmitt, president of Schmitt Technical Services in Madison, Wis., a consulting firm that specializes in chemical analyses of geological and construction materials. “Yes, weaker areas of the plaster are more likely to develop spots — and aggressive water is more likely to attack those weak areas. These causes and effects influence one another.”

We typically say that etching is a form of deterioration that involves a preferential dissolution, which means there are weak links in the microstructure that are more easily dissolved than others. Some reports may choose to define those terms differently, but going by the usage I usually see in the scientific literature, I’d say that all three are aspects of the process of water attacking cementitious materials.

As far as causes, it’s my opinion that water chemistry is an important factor in spotting — and so is the quality and workmanship of the plaster. It’s all about the interactions between these factors, rather than just one or the other.

— David Rothstein, president, DRP Consulting Inc. , Boulder, Colo.

In short, some scientists are reluctant to assign the blame for plaster spots exclusively to aggressive water or to plaster problems. Depending on the specific case, they say, either or both may be the determining factor.

“I absolutely think water chemistry, plaster mixes and plaster application can all play a part in causing these spots,” says Laura Powers, associate principal at Wiss, Janney, Elstner Associates Inc. in Northbrook, Ill., a firm specializing in structural, architectural and materials analysis. “When I study core samples of spotted plaster, I can’t say that the microcracks I see were caused by water chemistry because those are clearly due to troweling. But I also can’t say that the secondary porosity I see is caused by troweling because that’s clearly a water chemistry issue.”

I wouldn’t say that these spots represent a qualitatively different phenomenon from other types of plaster etching. They’re all results of calcium being pulled out of cementitious material by aggressive water.

This isn’t a simple black-and-white issue. Pool water can be quite aggressive toward plaster, and can etch it and produce soft spots. But, at the same time, weaker areas of plaster will be more susceptible to attack by aggressive water. Spotting can be due to one or the other — or to both.

— Robert O’Neill, senior petrographer, Micro-Chem Laboratories , Murphys, Calif.

Thus, instead of drawing absolute distinctions between processes that are often interrelated, experts suggest that a more scientific solution is to take each case on its own terms, working to assess what mixture of causes led to spotting in that specific circumstance. And in many cases they’ve seen, they say, there’s plenty of responsibility to go around.

The score

It’s easy to understand why some service techs blame spots on faulty plaster workmanship, while some plasterers want the burden to fall on imbalanced water chemistry. After all, at the end of the day, someone’s got to foot the bill for the damage.

But the truth may not fall neatly into either camp.

One common assumption about the scientific method is that it sets out to prove, or disprove, hypotheses, but this isn’t always true. Science often works by searching for exceptions to a predicted outcome by reanalyzing data and performing new experiments. The day-to-day work of many scientists is far more focused on disproving existing ideas than on proving new ones. For that reason, many scientists regard dissenting voices as signs of healthy progress, but only insofar as those voices themselves are as willing to overturn their convictions as they expect their peers to be. The annals of science are littered with the ruins of inflexible “all-or-nothing” hypotheses that weren’t adaptable enough to permit exceptions.

This is why, while many scientists agree that plaster workmanship is a contributing factor — sometimes a major one — in cases of spotting, the hypothesis that aggressive water chemistry is never a factor just doesn’t fit neatly with all the data they’ve examined.

It’s important to recognize that no one’s saying the blame should be split 50-50 in every case. Instead, scientists believe that because both water and plaster conditions can contribute to spotting, the blame sometimes falls on the service tech, sometimes on the plasterer, and sometimes on both.

When I look at spotted plaster surfaces and I see cracks parallel to the outer surface of the plaster, it tells me those cracks were probably formed by the troweling operation — and when I see unusual porosity in the plaster, that tells me that water was attacking the cement paste.

All I can do is try my best to report what I’ve found in the lab by using a scientific approach.

— Laura Powers, associate principal, Wiss, Janney Elstner Associates Inc., Northbrook, Ill.

This concept of comparative fault isn’t just an abstract scientific idea: It’s well known in the legal realm. In many cases where damages have occurred, the court assesses which of the negligent parties should be responsible for reimbursing which percentages damages to the plaintiff. By the same token, regardless of what form plaster damage takes, it’s necessary in each new case to establish to what degree (if at all) the water chemistry was out of balance, to what degree the plaster was improperly mixed and/or troweled, and to what degree each of these factors contributed to the overall damage.

“That,” Schmitt says, “is how you help keep everybody playing on a level field.”

Taking precautions

If neither poor plastering nor water chemistry is necessarily to blame for spotting, what steps can a tech or a plasterer take to protect against liability? Again, the answer lies in hard data, and scientists agree that an ounce of prevention adds up to a pound of cure.

“If a plasterer wants to protect himself, he should make two plaster coupons at the same time he’s plastering the pool, and put them in storage,” Schmitt says. If a damage case escalates to the point that a consulting firm is called in, the chemical composition of these coupons can be analyzed under laboratory conditions.

While the plaster coupons might have been differently troweled than the areas of the pool shell that acquired spotting, they still provide valuable data on the plaster’s chemical makeup.

For a service tech who notices spotting on a pool, onBalance’s advice is to take pictures, and communicate to the homeowner in writing. Carefully noting and saving water chemistry test results can provide additional evidence of proper balancing on the tech’s part. However, to show a history of properly balanced water, those tests must have been performed well before the spotting was ever detected.