Stain Removal in a Plaster Pool

pool-cannonballSure, your inground swimming pool is a resort-like escape where you work on bronzing your skin poolside while children splash about on giant, inflatable alligators, and immature friends tirelessly scream, “CANNONBALL!

It’s also one giant chemistry beaker perpetually on the brink of bubbling over with green algae blooms or suddenly breaking out with colorful stains as your friends slowly back away from water and ask “Eeew, what is that stain? You must be doing something wrong with your pool.”

If you own a concrete or plaster swimming pool, or are about to become the proud owner of a plaster-surfaced pool, we have some pointers to help you keep it looking like paradise and not like a 20,000 gallon petri dish.

Plaster pool finishes are traditionally the most popular swimming pools dating all the way back to the very first backyard pools. Typically a mixture of cement and fine, crushed marble sand, otherwise known as marcite, it’s a durable, impact resistant surface.

plaster-pool-finishPerhaps best of all, they can be sculpted into almost any shape your budget can afford. Additionally, gunite pools are very sturdy and strong because of the steel framework.

However, the plaster finish on a gunite pool needs to be replaced about every 15 years and that can get quite pricey (over $5,000) for a surface that is a bit rough on the skin due to it’s natural, porous nature. Oh, yeah, about that porous surface…

Smoother surfaces, like vinyl or fiberglass, are less prone to stains because they do not lend themselves as easily to cozy little nooks and crannies for metals, oils, or organic debris to eventually coagulate into an unsightly, rash-like stain on your pool plaster.

Plaster pool surfaces create their own vicious cycle – because plaster is so porous, more cleaning is required to prevent algae growth, and the high alkalinity and pH of plaster requires frequent adding of acid to counteract the effect which in turn, of course, slowly deteriorates the plaster surfaces that you are trying so hard to keep clean – making them even more porous which makes them more susceptible to stains…You can see where this is going.

Let’s get on with it – how to remove stains in a plaster pool, and the best methods to prevent plaster pool staining.

Heavy metal is the enemy, and I am not referring to Iron Maiden or Slayer. I’m talking about the invisible metals dissolved in your pool water from sources like rain run-off, pool equipment, salt water pool systems or even your concrete pool deck surface. Bits of metal like screws or hair pins can make a nasty rust stain.

Natural Chemistry's Metal FreeA great product for removing metal from your pool is Metal-Free by Natural Chemistry. It is one of the strongest products available for sequestering metals. It works especially well on well water, is not affected by pH or temperature, and is All Natural.

sapphire-stuff-by-jacks-magicThe Sapphire Stuff by Jack’s Magic is another fantastic sequestering agent. It not only removes metals but also oils, soaps, cosmetics and other organic matter from your pool water.

pool-perfect-by-natural-chemistryFor oily stains at the waterline, or a bathtub ring from winterization water levels, use an enzyme product like Pool Perfect to consume oils, fats and other greasy pollutants that enter the water.

What’s that Stain?

It’s important to know what kinds of metals are creating the stains in your pool. Here’s a pool stain chart as a guide for metal stain identification in pools.

color-chart-for-metal-stains-in-poolsMetals are certainly not the only cause of stains in a plaster-surfaced swimming pool. Leaves, berries, bugs, sun tan oil, or even your water-loving dog can all contribute to staining. These types of stains can be cleaned with good old fashion elbow grease using a combination of a PoolStone and pool shock. Pouring a bit of pool shock directly onto a stain works similar to bleach and is ideal for plaster surfaces but a bit harsh for vinyl liners.

Stain EraserThe Stain Eraser is another great stain removal product that does not require the use of chemicals and is great for smaller stains and stains in tough corners. It’s designed to be mildly abrasive so it’s tough but not destructive to the plaster.

acid-magic-galFor extreme staining or serious algae blooms, plaster pools can be cleaned with an acid wash with muriatic acid, or the safer to use Acid Magic. Acid washing removes a thin layer of plaster, exposing bright white and unstained plaster underneath. Having your pool acid washed will remove most surface stains.

Finally, keep your pool water balanced, with good pH, alkalinity and calcium hardness levels. A pool that is out of balance can stain more easily, and makes stain removal difficult as well.

Removing Stains from a Plaster Pool

  • Granular pool shock for organic stains
  • Sequestering agents for metal stains
  • Stain Eraser or Pool Stone for mineral stains
  • Enzymes for oily, dirty stains
  • Acid Washing for all stains
  • Keep pool water balanced


Ryan Dornan
InTheSwim Staff Blogger



Stain Removal in a Plaster Pool — 27 Comments

  1. Pingback: Stain Prevention in a Plaster Pool | InTheSwim Pool Blog

  2. Davy,

    This is the best blog we have come across. Direct answers to the problem at hand. We have 20 year old plastered pool that is in good shape, except for the staining from years of wear and leaves from our sycamore trees in South Carolina. From your blog, we need to acid wash the pool and adjacent spa to rejuvenate the surfaces. What about coatings to take it from the 90’s white to a color more appropriate for today.

    • Hi tom and thanks – yes, acid washing will whiten and brighten the finish, but can only be done so many times, before it wears thru. Like Model T cars, it used to be that all pools were one color, white. Then black, grey and blue plaster (now tan too) became popular, along with pebble finish with small pea gravel sized aggregates. Then 3M introduced colored quartz aggregate additives, and from Australia we have glass bead aggregates (very small), all of which add a hue and sparkle to the pool, and don’t show staining as much as white pools, or colored plaster pools. Pozzolans can also be added to plaster for added strength, etc… So now there are many options when it comes to replaster. White plaster is usually around $4000 and the additives can double the price, for an average pool, in my area. About every 20-25 years, a pool does need a new plaster coat, and often new tile and/or coping is done at the same time, for a complete renovation. Tile and coping cost another $5K each, or more if you go for the fancy stuff 😉

  3. Hi, I don’t really have a stain problem per say but I can see the plaster looks slightly dirty in general on the walls, bottom and I can see streaks where the sweeper has moved around. I think this was from some heavy winds we had and then my sweeper was out for a couple of days too. It’s not something I think needs a harsh acid wash. I don’t shock my pool normally but maybe it needs it just to clean up the surface. What would you recommend to remove the dirty film?

    • Hi Dee, oftentimes a dirty film is a calcium scale, mixed with dirt. You can try a good shocking first, but you may want to use our EZ Stain remover, and acidic based cleaner that will dissolve scale. Lower your pH before using to 7.2, and if your calcium hardness is high (over 400 ppm), use a sequestering agent like our Stain Away a few days prior, to keep calcium in solution, and run your Alkalinity in the low end, 70-80 ppm. All plaster pools have some sort of mottling or uneven hues, and many (most?) plaster pools also have calcium scale and film, not always visible on white pools, and more visible in the right light. The stuff above should do the trick, follow label directions, if not then an acid wash may be in order, which also gives the benefit of having fresh pool water for several years.

  4. I have some state names in the bottom of my plaster pool from mud that was left behind after Hurricane Harvey. How can I get these satins off? It is a new pool.

    • Hi Tracy, start with a heavy shocking, if you haven’t’ done that yet. 2 lbs of granular shock, per 10,000 gallons, sprinkled over the areas – then brush well, really well, for several days. If that doesn’t work, try EZ Stain Remover, following directions on label. If THAT doesn’t work, then you may have to drain and acid wash, although the stains will continue to lighten and perhaps disappear on their own, over a few weeks/months…

  5. I definitely have extensive metal stain ( blue-green and likely from my heat pump installed 15 years ago when pool was refinished with Crystal Crete). After consulting my local pool gurus and spending $ I was able to remove the stains with intensive monitoring. Now the the “use season” is here the stains are redepositing on the surface ” in spades”. I would like to avoid a drain and acid wash – is there any way to remove the disassociated metal ions from the water? If not chemicals seem like a waste of time and money. KNOWLEDGEABLE answer would be appreciated.

    • Hi Dan, you can control the metals using sequestering agents, to keep the metals locked in solution. These need to be added regularly – every week or two, because they degrade quickly. Products like MetalFree or Stain Away – any Stain & Scale chemical. Secondly, there is a product called PoolRx that traps metals in a pouch that sits in the pump basket, to actually remove ions from the water.

  6. I just bought this older home. I have an older in ground plaster pool which has some “stains” on some of the steps and the bottom of the pool. I have tried to use some shock and chlorine and scrub to see if that would remove the “stains”
    Any other suggestions? Thank you


    • Hi Jason, shock will remove most organic stains from leaves, worms, algae, but won’t affect mineral stains or metal stains, or areas where the plaster is so thin that the gunite is showing through. When chlorine won’t remove a stain, the next step is to try an acid. You could sprinkle dry acid over the area, or create a ‘stain bag’ using dry acid or EZ Stain Remover in an old sock (finally a use for those orphans). Or you could pour a few cups of muriatic acid into a PVC pipe that you hold over the stain, and slowly move the pipe, while keeping it tightly contacting the floor, or surface. Attach a 90 fitting and you can do vertical walls too. Wear goggles and gloves if you try this, and afterwards you will likely have to raise the pH level back up. A Jandy Stainmaster is another option, which delivers muriatic acid or other liquids to precise underwater areas with a special cup that attaches to a pool pole.

  7. I have a new light color River Roc plaster that’s less than 2 weeks old. A piece of mud/dirt debris went inside the pool, I tried to rub it off with finger, in the process I ended up spreading the stain and rubbing the brownish color into the plaster. I tried brushing it out using shock and chlorine tabs… but the stain did not even budge.

    • Hi JL, you can use a device called a stainmaster, which connects to the pool, and has a long hose to connect to a gallon of muriatic acid on the deck. After priming the hose, it will release acid directly to the stain, inside the 3″ dia. cup. That should do the trick. Alternatively, if not in the deep end, take a 4′ piece of PVC pipe, 1.5″ or 2″ dia., and pour in 1 lb. of pH decreaser, into the pipe, while holding it on the stain. Wait 60 seconds and slowly move the pipe an inch or two, around the stain, letting the acid work on the stain. Another way to do this is to add pH decreaser, or EZ Stain Remover to an old orphan sock (finally a use for those!), and tie off the sock, cut off the excess, and place it over the stain with your pool brush, and move around as needed.

      • Thanks for your ideas, I think I will try the stain remover as it is on the steps under only 6″ of water. Do I do this under water as is or should I pump out the 6″ of water first and do it dry?

        • If it’s only 6″ to expose the stain on steps, I would try to acid wash a (moist) surface first. Add 1 cup of muriatic acid (or 1 lb dry acid) to 1 cup water, and drizzle or pour over the area. then use a scrub brush or push broom to keep it over the area and scrub a bit, then hose off fully, within 60 seconds.

  8. We have a plaster pool and ducks crapped all over the bottom leaving black stains..Any suggestions on how to remove them??

    • I would first balance the pH to the low side, 7.2-ish, then pour shock chlorine over the stains, let it sit and then brush vigorously. If no change, fill an old orphan sock with a pound of EZ Stain Remover, and place it over the spots for a few minutes (pH decreaser can also be used), move it with a brush to the next location. Either, or both methods should work to remove bird poop stains

  9. We have a newly painted gunnite pool (2016). We had a problem with yellow algae. We were able to brush the algea away and then put in algaecide. A couple of weeks ago, I added calcuim up and alkinity up the next day to the pool, but did not scrub the algea from the walls before adding the chemicals. After adding the Chemicals, the algea seemed to bond to the pool sides and walls and has now stained the walls and bottom. What is the best treatment for stain removal without draining the pool and acid washing? Could adding the calcium caused the algea to bond to the pool?

    • Hi Millie, there could be a calcium Film that attached itself to the pool walls, locking in the stains. First thing I would try is to drop the pH to 6.8 – testing with an accurate kit, to compute acid demand to get precisely to 6.8. Brush the pool two or three times thruout the day, and after 12-24 hours or so, bring the pH back up to 7.2, using a Base Demand test, or other accurate means of calculating the pH increaser dose. The paint won’t like the low pH, so don’t keep it there for too long. Other methods could be using ascorbic acid, in StainFree or our A+ Stain Remover products. Or EZ Stain remover, another mild acid that could be successful on ‘Film Scale’. Good luck! If you do drain the pool, and acid wash, use a mild solution, just 1 part acid to 10 parts water. Go stronger if needed, but test first with mild.

  10. Hi, we just opened our pool and the winterizing tablets leaked out of the tubes and stained our plaster pool white where the residue landed. How can we get that stain out? It’s at the bottom of the pool. Please help. The pool was plastered two summers ago.

    • hi debbie, you can try a stain sock, using EZ Stain or A+ Stain remover, putting 1 lb into an old sock and place it over the stain, move it around after a few minutes with a pool brush. If that is not strong enough, a product called a Jandy stainmaster can be used to deliver liquid acid directly to the area. These types of stains (from chlorine tabs) can be difficult to remove completely, but can usually be mostly removed, or lightened considerably.

  11. Hi I have black stains at the bottom of the pool in the exact shape of where the powdered shock settled. How would I get rid of those stains? I tried brushing with a metal brush but it wont come out. Someone recommended a pumice stone? But how do I get it all the way down there the pool is 10ft deep. Someone else said I have to drain the pool and acid wash it.

    • Hi Madina – Sounds like it could be metals dropping out of solution. I have had this happen to me before, on customers’ pools where we were using too much copper and/or silver algaecide, combined with a mineral purifier… anyway just like you said, shock the pool and then there appears instantly this dark staining across the surface where the shock feel, real shadowy looking. to my surprise and joy, they disappeared overnight. If that doesn’t happen in your case, treat with a Stain Away type sequestering agent, which may put the metals back in solution, and lift the marks. If that doesn’t work, try Stain Free treatment. If THAT doesn’t work, maybe a drain and clean, yes. But give it a day or two, check and balance the water…

  12. I added ProTeam UV Enhancer and apparently not very well since there are white round stains on the bottom of the pool where I dumped it in. Looks like it didn’t dissolve and sat at the bottom and stained. I have only had the pool for about 6 weeks. Anything to remove the stain or will it eventually go away?

    • Hi Neil, UV Shield (enhancer) is a compound of cyanuric acid, or chlorine stabilizer. I have not known cya to bleach pool surfaces, but their product likely has other chemicals in it. Cyanuric acid also gives off a bit of heat when it dissolves. I’d start with agressive brushing of the area, with a steel brush if you have one (and you don’t have a vinyl pool). Check and balance the water chemistry and then pour granular shock over the area (again for plaster pools only). If your pool is a dark plaster color, you can then try an acidic stain remover like Stain Free or Stain Away.

  13. I have one little spot at the bottom of my pool that the gunite shows through the plaster of the pool. It is small and not a big deal but I would like to cover it with something if I could just for cosmetic reasons until I have the pool re-plastered. Do you have any suggestions?

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