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
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 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
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.
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
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
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
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
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
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.
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
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
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.”
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
“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
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.