Even a well-scrubbed pool can fall victim to metal or mineral deposits every so often.
These blemishes might form around the waterline, or harden along
the steps. And because their sources can range from underground
mineral layers to popular chlorine compounds, the process of
tracing scale to its source — and preventing its return
— can seem like a daunting task.
The good news, however, is that scale and encrustation have been
studied by geologists, service technicians and plasterers for
decades. And the solutions they’ve developed are often
surprisingly straightforward. Here, experts present their steps for
diagnosing, removing and thwarting the return of scale.
Peering down into a pool, it may be hard to tell much detail about
the composition and causes of scale. This leads to popular
diagnoses like “high calcium hardness” or “high
total dissolved solids.” And while it’s true that
scale’s roots ultimately lie in excess calcium, the only way
to be sure of its cause — and the proper method for treating
it — is to test the chemistry of both the fill water and the
water in the vessel.
“I determine the source of scale by comparing the mineral
content, calcium hardness, pH and total alkalinity of the source
water with those of the water in the pool,” says Pat Fay,
owner of Pat’s Pool & Spa Inc. in Manhattan, Kan. In
fact, Fay says, a single comparison is rarely enough — he
strongly recommends returning to the job site every other day for
at least 10 days, and comparing all sets of samples against one
This may seem a tad obsessive — and indeed it would be if
fill water chemistry could be trusted to stay consistent from block
to block, and from day to day. But the fact is that many cities
supplement their municipal water supply with water from wells
across the county, which means two pools on the same municipal
water system often have completely different fill water chemistry
from one day to the next.
Comparing the calcium hardness, total alkalinity, total dissolved
solids and pH of the fill water and pool water across several days
helps provide a clearer picture of what’s causing the calcium
to precipitate out of solution and form surface deposits.
For instance, a certain pool might contain excessive levels of
calcium — but if the alkalinity and pH are low enough, that
calcium may never precipitate out of solution and harden into
deposits. On the other hand, the water’s conditions may be
just right for scale to form — but until a certain level of
calcium is actually introduced, the pool might stay scale-free. So
it’s crucial to understand exactly what chemical conditions
are creating a scale-friendly environment in the pool.
Next, take a closer look at what’s being added to the water,
and where it’s coming from, in order to determine what
combination of chemical factors are promoting scale.
Trace supply lines
This is where those tests on fill water and pool water really come
in handy — a careful comparison of the two types of water
over time will help determine whether calcium and other
scale-promoting chemicals are entering the vessel from the source
water, or if their origin lies elsewhere.
“If the fill water has higher alkalinity or calcium hardness
than the water in the pool, you can try to lower your total
alkalinity in the pool to compensate for that,” Fay
Still, it’s important to keep the water’s overall
Langelier Saturation Index value balanced within a safe range
— at the very broadest, -0.3 to +0.5 — recommended by
groups such as the Independent Pool & Spa Service Association
and the Association of Pool & Spa Professionals. (For a
detailed walkthrough on calculating the LSI, see the article
“Saturation Calculation” in the July 29, 2011, issue of Pool
& Spa News.)
If the problem has been traced to fill water, treating it is simply
a matter of adjusting that water’s chemistry before it enters
the pool. This can be accomplished with an in-line softener, a
stain and scale control chemical treatment, or some combination of
the above — each pool is best treated with an approach that
accounts for its unique chemical makeup.
If, on the other hand, the source water contains less calcium than
the water in the pool — or if the two calcium levels are
roughly the same — take a closer look at what chemicals are
being added to the vessel. If the customers are sanitizing with
calcium hypochlorite (commonly known as cal hypo), they may also be
adding significant doses of calcium to the pool, especially if it
was recently treated with shock.
In rarer cases, elevated phosphate levels can contribute to the
formation of calcium phosphate, another chemical compound that
creates calcium scale. If water testing indicates the presence of
high phosphate levels, clearing them out with a flocculent or other
phosphate remover will help prevent the scale from
Clear it out
Whether the scale’s formation is due to hard source water,
calcium-based chemical treatments or some other factor, the
strategies for removing it follow a clear, step-by-step
Start with the gentlest method possible, and step up the onslaught
until the scale begins to disappear.
The first line of attack should come from chemical adjustment
— if the fill water’s chemistry isn’t a major
contributor to the problem, a simple lowering of the pool
water’s pH may be enough to help the scale break down.
The next step up is a mild acid wash: One part muriatic acid to 10
parts water (always add the acid to the water — never the
other way around). Just pouring the acidic mixture onto the scaled
area and letting it sit for a few minutes is often enough for the
scale to begin flaking off. In other cases, it may be necessary to
rub the spot gently with a rag or brush.
Many chemical manufacturers also carry scale removal kits that
include specialized agents for breaking down calcium deposits.
These can be a bit pricier than muriatic acid, but they may do the
trick more effectively, and prevent the need for more aggressive
In tougher cases — for instance, when the scale’s had
several seasons to build up a thick crust — it may be
necessary to apply a bit of elbow grease. Some techs say sandpaper
works wonders, while others recommend a soft but steady scraping
with a metal-bristled brush.
If the scale doesn’t respond to the treatments above, a more
extreme option is to drain the pool to a point below the scale, and
attack the buildup with a belt sander, and perhaps a more
concentrated bath of muriatic acid. This method is best saved for
emergencies only — not only is it costly, but it risks damage
to the pool’s surface if not done with extreme caution.
In short, the safest bet for scale treatment is to start simple and
gradually work your way up.
“We always try chemical methods first,” says Brian
Diglio, president of Blue Wave Pool Service and Supplies Inc. in
Hamden, Conn. “Then we move up step-by-step to physical
methods like acid washes and sanding, until we get to a method
Keep it out
Once the scale is removed from the pool, there’s no guarantee
it won’t return. Scale has a tendency to creep back onto
surfaces during winter closings, when calcium accumulates in the
water, and during droughts, when wells bring up calcium deposits
from deep underground.
Still, a simple chemical regimen and a careful eye for trouble
signs often can prevent calcium deposits from taking hold once
Most techs agree that the best way to protect against future
scaling is through regular testing of both the fill water and pool
water — and vigilant attention to the water’s pH and
“Chemical balance is the most important concern for keeping
scale from coming back,” Diglio says.
Even if testing reveals that the fill water’s chemical
content is contributing to scale, tailor-made solutions are
available. As mentioned above, some techs have solved the problem
by installing a softener on the fill line, while others have run
the pool’s pH slightly lower than usual.
Ideally, though, try to treat the hardness of the source water
before it enters the pool.
When it comes to the pool water itself, many chemical manufacturers
offer sequestering agents specially designed to clear out scale of
certain composition — calcium carbonate or calcium phosphate,
for instance. Others are formulated to work under specific water
conditions, such as low pH or saltwater.
“I commonly stock five different sequestering agents,
depending on what I’ve found, over the years, to work the
best with the water conditions I encounter,” says Phil
Helland, owner of Crown Pool and Spa Service in Portland,
Of course, none of these solutions by themselves will ensure a pool
stays scale-free. But with a thorough understanding of the source
of excess calcium, a grasp of the chemical conditions that promote
scale buildup, and the patience to see a treatment through to
completion, it’s often possible to restore a scaled pool to
polished perfection — and keep it that way.