How to Remove Stains from Pool Plaster

April 20, 2021

Taking care of your pool requires time and attention. While weekly pool cleaning can help clear debris, pool plaster stains will require a more targeted approach.

A variety of different types of stains can appear on your pool plaster over time. If you notice your pool plaster is beginning to stain, consider the following steps.

In this article, we’re exploring how to remove stains from pool plaster as well as prevent stains in the future.

How to Remove Stains from Pool Plaster

Pool plaster can be stained by several different organic and inorganic materials, each of which requires its own cleaning solution. 

Sometimes, the brush alone won’t do – especially when the stain has had time to work itself into the surface. So how do you remove stains from pool plaster? You start with a little diagnostic work. 

Here’s how to remove stains from pool plaster and prevent stains from appearing in the future.

how to remove stains from pool plaster

What Causes Pool Plaster Stains?

The first step to removing stains from pool plaster is identifying them. Pool plaster stains are always either organic (think algae, stains left by fallen fruit, leaves, and flowers that have settled and decomposed), or inorganic (think metal and mineral concentrations from eroded soil, well water, accumulated rainfall, rusty garden installations, and more). 

Organic stains and inorganic stains each require a different course of action. Most pool stains are ultimately treated through a rigorous scrub – but first, you need to chemically treat the spot to work the dirt and pigments out of the plaster. 

Color is Key

Most blotches can be distinguished by their color. This isn’t an absolute test of whether a stain is organic or inorganic, but it can give you a pretty good idea. In general, the common colors to look out for when treating pool stains are green, bluish-green, reddish brown, and purple.

Green Stains

Greenish stains are usually organic in nature. Especially when they tend towards brown. 

Blueish-green Stains

Blueish-green stains can come from certain plant matter, but if you don’t have a berry bush or bluish-green plants nearby, then the cause might be corroded copper from copper piping in your plumbing. Copper can also be present in municipal or well water. 

Brown-reddish Stains

Brown-reddish stains are usually indicative of iron, especially rust. This could have flown into your pool after rainfall from a rusty gate or corroded fence. 

Blackish-purple stains

Blackish, purplish stains are indicative of manganese or copper cyanurate. This is another mineral common in well water and some municipal water. While municipal water is treated for manganese, it can still be found in some concentrations. The same goes for copper. 

Use a Chemical Test to Determine the Stain

Color alone isn’t always the clearest indicator, and you should generally use a chemical test kit to confirm your suspicions before determining your shopping list for the stain. 

There are special organic and inorganic test kits online, but the easiest way to figure out if you should bother to buy a test kit to begin with is by taking a little granular chlorine, dropping it into the pool onto the stain, letting it take effect, and then grabbing a brush

If the chlorine makes the stain fade away and completely disappear with a light brushing, then it was an organic stain. 

Metal stains don’t usually react to simple chlorine treatment. In fact, shocking a pool excessively to fight a metal stain can cause more staining, as it causes metals to drop out of the water and settle on the plaster. 

Removing Organic Stains from Pool Plaster

Organic stains can be removed through chemical shocking and frequent brushing

Step 1. Check Chemical Balance

Start by ensuring that your pool water is properly balanced. Pool supply stores sell test strips for cheap, letting you quickly assess your water’s pH levels and alkalinity and ensure that they are within acceptable parameters. 

Step 2. Measure Water Volume

Figure out your pool’s total water volume. There are online calculators to help you figure out how many thousand gallons of water your pool holds based on its shape and dimensions. 

Step 3. Add Shock Chemicals

Add pool shocking chemicals (granular oxidizing agents) equivalent to the severity of the staining. Smaller stains can be removed with a single dose (about a pound per 10,000 gallons of water) – severe staining may require a double or triple dose. Do so after dusk. Sunlight breaks down hypochlorite, releasing chlorine into the air. This can affect the shocking process. Shocking your pool as soon as the sun is down can maximize the effectiveness of chlorine. 

Step 4. Brush the Pool Plaster

Let the pump run overnight to circulate the shock, and brush intermittently every few hours. If the stains aren’t completely gone the next day, rinse and repeat.

Once the stains are gone, you can use the remaining test strips to ensure your chemical balance (alkalinity, calcium hardness, pH, and chlorine content) is returned to normal.  

Removing Metallic Stains from Pool Plaster

Different inorganic compounds require different chemical treatments. Ascorbic acid can be used to bleach the pigment in the stain, but you might need to buy a pool metal test kit to figure out exactly what metal you’re dealing with (in order to buy the appropriate metal stain remover). 

Metal stain removers each have different instructions associated with them, but most require you to lower your pool’s pH (with muriatic acid), lower your chlorine levels, use the product, brush afterward, and finally rebalance your pool chemistry.

Metal stain removers tend to be chelating agents, meaning that they bind to metal ions and pull them out of the plaster. Some stain removers double as sequestrants to reduce the likelihood of metal staining in the near future. Note that metal sequestrants alone do not usually remove a stain and are meant for preventative use if your pool is regularly exposed to inorganic material. 

Tips for Preventing Pool Plaster Stains

Once you’ve identified the source of the stain, you can figure out what garden elements to keep an eye on to avoid having to remove stains from your pool plaster in the future. Sometimes, just getting out in front of the problem (by doing some pool landscaping, more yard work, or regularly skimming your pool) can greatly reduce the risk of staining. 

Some sources of metal cannot be avoided around the pool, including stone water features and nearby rock formations, or metal concentrations in the soil. Marble and granite are naturally high in iron, which can cause staining after hefty rainfall (rain is naturally acidic, at a pH of 5.5-5.0). 

If you’re facing particularly stubborn stains or are regularly plagued by pool stains appearing out of nowhere, and aren’t sure how to tackle them, consider seeking professional help. Pool experts may be able to locate additional sources of staining in and around your pool area, and help you take measures to reduce the risk of staining your pool plaster in the future.



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