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.

Compare chemistry

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 another.

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 says.

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 re-forming.

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 process.

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 methods.

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 that’s effective.”

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 again.

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 total alkalinity.

“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, Ore.

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.