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Thread: Gray Mottling Plaster Discoloration

  1. #1
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    Gray Mottling Plaster Discoloration

    Gray (also spelled “grey”) or darkened blotchy or streaky discoloration of pool plaster (cement flatwork) is known to be caused by several improper plastering practices, including the addition of calcium chloride to the mix (even less than 2%), late hard or dry troweling, and sometimes a cement material issue can lead to this problem. Trowel “burn” is another cause of extreme dark discolorations of white plaster. It is also called “pinto concrete” in the cement industry. (Concrete Slab Surface Defects: Causes, Prevention, Repair Portland Cement Association 2001).

    Although some discolorations are the result of metals precipitating and depositing various color residues onto the plaster surface, these are referred to as surface staining. Usually, this type of staining can be easily and safely removed from the surface by acid washing, sanding, using sequestering or chelating chemicals, or other stain removal products.

    For this article, we are discussing a blotchy or streaky, and very smooth gray discoloration that sometimes develops during the first few months after plastering and being filled with water. This graying discoloration is difficult to remove, and is often termed as a “hydration problem” or “entrapped moisture.” This type of dark discoloration should not be confused with metal or calcium scale staining.



    Late hard troweling can cause a darkening of the color of white cement because it decreases the water-to-cement ratio of the plaster surface. A very low water-to-cement ratio almost always leaves a darker finish color. The addition of calcium chloride to a plaster mix is also known to cause graying of cement, in addition to making the plaster less durable over time.

    Gray mottling of new plaster pools is sometimes accompanied with “white soft spots” or streaking (also incorrectly called “spot etching” or “etching deterioration” by plasterers). Research by independent cement laboratories have documented that white spotting and streaking is not the result of an aggressive water (etched caused) condition, but is caused by improper workmanship practices and troweling techniques (such as adding lots of water while troweling) which results in localize areas of greater porosity (lighter soft areas or spots) on the plaster surface.

    Some plastering people incorrectly claim that aggressive water, high cyanuric acid levels, or improper water chemistry startups causes the graying discoloration. There is no research study that supports their theory. Sadly, these incorrect claims enable some pool plasterers to avoid responsibility for improper practices, and place blame onto innocent pool owners or service techs.

    In an attempt to remove the smooth gray discoloration, it is sometimes suggested to perform a “zero alkalinity process” or an acid treatment on the plaster. That is the wrong thing to do. Acidic water treatments are an undesirable and improper suggestion for the service tech or pool owner to perform on a new plaster job. While this treatment may occasionally lighten gray discolorations, it often doesn’t work, or the gray color returns within weeks afterwards. But the most unfortunate issue is that the plaster surface WILL BE ETCHED after the acid treatment process is performed, which ages the plaster surface and allows for staining to occur much easier and sooner.

    If an acid treatment is used to lighten gray mottling, how is it that slightly aggressive water caused white plaster to darken and turn gray, and in a blotchy, streaky, or spotting fashion in the first place? If the gray discoloration isn’t removed by the acidic process; it will then become difficult proving that it wasn’t aggressive water that caused the gray discoloration in the first place and the pool plasterer may be able to avoid being held responsible.

    Sometimes, “torching” (heating) the gray area with a propane torch device is performed in an attempt to remove the gray color. It is unclear whether the torching process affects the long-term durability of a plaster surface, but there are studies that indicate some damage is done to cement products. It is known that the plaster surface may “pop” when heated to such high temperatures, so protective goggles should be used. Be aware that the above remedies may only be temporary as the gray discoloration may return after the pool is filled again with water.

    There is also normal mottling of plaster where the white plaster surface has very slight shade differences from one white area to another. Again, this slight difference still has to do with the fact that plaster is made of a mixture of water, cement, and aggregate, and is a hand-crafted product. At its best, there will still be a slight variation.

    As one can see, all of the above discussions regarding dark discolorations of white plaster has to do with workmanship, additives, hydration problems, and/or curing issues, not about water chemistry and whether the water is aggressive or not. Aggressive pool water does not cause a darkening of white cement, nor does it detrimentally affect the hydration rate of the cement compounds.

    See also this post: ten-guidelines-for-quality-pool-plaster-t42957.html
    Last edited by onBalance; 06-04-2014 at 09:48 AM. Reason: Additional information

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    Re: Gray Mottling Plaster Discoloration

    Can you look at my post and advise? On day 5 after the pool was replastered, I put the hose in the pool to add more water, as it was getting close to the bottom of the tile, and now there is a big area that looks like it was "washed clean" from the hose spray at the bottom of the pool/deep end. Can I fix this? Will this go away in time and match the rest of the pool?

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    Re: Gray Mottling Plaster Discoloration

    Tldoige's pool has light colored streaks. It is common that plasterers use water while troweling which leads to this type of lighter discoloration on dark or gray plaster. Adding calcium chloride to the plaster mix also plays a role in this.
    Unfortunately, trying to darken up the light areas is difficult, sometimes a no drain acid wash works, but draining and sanding works better, but is more labor intensive. Hard call. Depends on severity.

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    Re: Gray Mottling Plaster Discoloration

    Thank you.

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    Re: Gray Mottling Plaster Discoloration

    Is it possible that the color will even out on its own?

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    Re: Gray Mottling Plaster Discoloration

    A large light colored streak generally will not disappear and blend in. It will need some corrective measures taken.

    Normal white pool plaster does usually mottle somewhat (blotchy lighter and slightly darker areas, like clouds), and will generally blend in after one year.

    Dark colored plaster mottles (light color streaks and blotchy) more severely and usually does not generally blend in.

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    Re: Gray Mottling Plaster Discoloration

    Thank you!

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    Re: Gray Mottling Plaster Discoloration

    Oh, how successful is the acid wash? If that doesnt work, what else can be done?

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    Re: Gray Mottling Plaster Discoloration

    There is a distinct difference between "gray mottling" discoloration on white plaster and a "light or white" discoloration on black, gray, or dark colored plaster.

    Gray plaster that has a light or whiten discoloration may be a white scaling issue, which an acid type treatment (either no drain or actual draining) would probably be quite successful in removing the white scale and darken that area again. However, if the light or white discoloration is due to the leaching of soluble cement material from the plaster surface, which causes a loss of the dark color, then an acid treatment is unlikely to darken those white and light areas. That is because the white areas are somewhat rough and porous, and an acid wash generally just makes it lighter and also will lighten the rest of the dark plaster somewhat. I am also opposed to doing acid treatments because it just ages the pool plaster. So unless there is a very good reason to do so, an acid treatment should be avoided if possible.

    Therefore, if the white or light discoloration is from cement leaching, then it is better to sand the whiten areas to creating a more dense and smooth surface again that will generally make that light area darker in color.

    And again, if the pool plaster is 5 days old and a water hose is placed on a diving board to add water to the deep end and is several feet away from the pool wall, it is very unlikely that that could cause negatively affect the plaster surface on a deep end wall. The exception is if the water "stream" impacted directly on to the plaster surface.

    A plaster surface is not soft after 48 hours of time. A plaster surface is very adequately hard within 24 to 48 hours. (Just make your own pool plaster coupon and see how hard it is after one day in water). Wheel marks caused by cleaning devices are not indentations. They are probably raised areas of plaster dust that is compacted by the wheels and then not brushed afterwards to remove and clean. Or they are simply some type of discoloration, but still smooth.

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    Re: Gray Mottling Plaster Discoloration

    I have a Blue Dimond Brite finish, one year old with major gray mottling. Mottling started within two months of filling pool and continues to worsen. Pool uses about a gallon of acid per month. Pouring acid on stain on steps doesn't do anything except create a little white cloud/dust. Help, what is the solution?

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    Re: Gray Mottling Plaster Discoloration

    Gray mottling is usually permanent. Did you call the plasterer when the discoloration started at the two month time? Did you take some pictures along the way? If it is indeed gray mottling and not rough calcium scaling, then you will probably have to prove to the plasterer that you have kept the water properly balanced for the entire year. The plasterer might recommend an acid treatment of some type to rectify, but I doubt it will work. An acid treatment often makes the plaster surface rougher (etched). Hopefully the plasterer will understand what the issues are.

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    Re: Gray Mottling Plaster Discoloration

    Thank you for your reply and original post about motting/pinto concrete (and reference to the PCA publications). This was the first promising lead I have found after searching for the past 8 months on what my pool problem might be (I tested for other possible staining causes but they proved not to be my problem), motting/pinto concrete seems to be the problem, but now what? (PCA pubs didn't offer a solution for pools.)

    I had notified the PB of the problem and was advised that brushing would/should solve it (over time). I have some pixs but they are not great. I have several water test results from different retail pool stores that do not show any adnormal metals/minerals, hardness, ph readings... I feel confident that it isn't calcium scaling (even my swg/chlorinator plates look like new after 10 months). I agree that an acid wash isn't going to resolve the issue as when I add acid I have been pouring it over the second step, and other than making it a little rough, it is still splotchy gray.

    I contacted the manufacturer of the finish product but no reply as yet and the PB said he will check into it (hopefully will hear back from them). Question is (if it is mottling) what will it take to fix it? Can the pool be sandblasted and refinished? I'm trying to figure out what the options/solution posibilities are and if, like plaster, a DiamondBrite quartz finish can be redone/restored.

    Thanks again for your posting and reply, greatly appreciated.

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    Re: Gray Mottling Plaster Discoloration

    If the discoloration problem is gray mottling/pinto discoloration, you will probably be fighting an uphill battle to get a good resolution. Yes, your pool finish can be redone properly. Depending on how much area is affected, replastering is probably the only way to get a non-discolored pool.

    It is good that you contacted the PB early on. You can show him the PCA publication (and there are others) on discolorations to help him understand the issues. But if there is resistance to rectify, you can have your quartz plaster analyzed by a professional cement lab to determine the cause. But that will require taking two plaster cores from the pool surface, one from a non-affected area, and one from the dark colored area. The cement lab can determine the amount of calcium chloride added to see if it is over the limit/standard, and also perform petrographic analysis on both cores.

    Also, go to the "Pool School" link on this forum, and then look at the bottom of page and find "Further Reading" under Miscellaneous, and read "Ten Guidelines for Durable Pool Plaster." That will help provide further information on the "do's and dont's" of proper pool plastering. But all of the above is dependant on whether or not your overall water balance has been good. A picture or two, current pool water balance readings, and tap water readings, would help with assuring us what the problem actually is, and then what to do.

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    Re: Gray Mottling Plaster Discoloration

    I appreciate the information on how to definitively diagnose the problem as well as knowing that there is a solution (if cal-chor mottling proves to be the problem), as well as what supporting evidence I need and where to find additional information. You do good work (you should be a teacher).

    I have learned that several other new pool owners in the area have the same complaint and that the PB has requested for a product company rep to visit/evaluate the problem. I'm sorry that others have to deal with this too, but maybe this will help avoid an uphill-battle resolution (or at least make it easier).

    Thank you again.
    I will post back when this is resolved (hopefully to give accolades to the PB and product manufacturer).

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    Re: Gray Mottling Plaster Discoloration

    Quote Originally Posted by onBalance
    Wheel marks caused by cleaning devices are not indentations. They are probably raised areas of plaster dust that is compacted by the wheels and then not brushed afterwards to remove and clean. Or they are simply some type of discoloration, but still smooth.
    Oh! so the dark stripes that I have in my light grey plaster pool may be from the pool cleaner? It runs 3 hours a day and the stripes do feel slightly raised and polished and are a deeper grey than the rest of the surface. I never brush the pool, or only in the areas where dirt settles when I must. So, it sounds as though if I were to begin brushing those areas with a SS brush, they may fade some? I have noticed that they have gotten worse in the almost 4 years we have been here.
    23,000 gallon in ground pool with rock waterfall and spillover spa, Aqualink control system, Polaris 380 cleaner, Purex Triton Clean&Clear Plus cartridge filter. Located in The Woodlands, Texas.

    Pool owner since Nov 2008, Trouble Free since April 2009. Happy to help when I can.

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    Re: Gray Mottling Plaster Discoloration

    Cshatzer, I agree that if other pool owners are having the same type of plaster problems, that will help with pressure and providing the necessary evidence to help get a good resolution for you and others in your area. I have been writing articles and teaching at various swimming pool trades show trying to inform Pool Builders, Plasterers, and Service industry members on pool plaster problems. I hope you will keep me informed on your progress, because I would like to be aware of what is happening in my industry. And I am willing to help you every step of the way. You can "PM" me here on TFP.

    Anonapersona, I have not seen pictures of your pool and the discolored streaks. Inasmuch as the color of your plaster is gray, it is plausible that plaster dust can darken over time due to dirt and metals attaching to the porous and hardened plaster dust (calcium carbonate). However, plaster dust is off-white in color during the first few weeks. Seems that you would have noticed white streaks during first few months. So I am not sure what would have caused the dark streaking of your gray plaster. Have you ever tried sanding the dark streaks (with 100 grit sandpaper) to see if anything is removed and exposing a different color underneath? Plaster dust "crust" is usually very easy to remove in this manner. In general, most dark colored plaster jobs lighten in color over time, not darken. So I am very curious about your pool.

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    Re: Gray Mottling Plaster Discoloration

    Quote Originally Posted by onBalance
    Anonapersona, I have not seen pictures of your pool and the discolored streaks. Inasmuch as the color of your plaster is gray, it is plausible that plaster dust can darken over time due to dirt and metals attaching to the porous and hardened plaster dust (calcium carbonate). However, plaster dust is off-white in color during the first few weeks. Seems that you would have noticed white streaks during first few months. So I am not sure what would have caused the dark streaking of your gray plaster. Have you ever tried sanding the dark streaks (with 100 grit sandpaper) to see if anything is removed and exposing a different color underneath? Plaster dust "crust" is usually very easy to remove in this manner. In general, most dark colored plaster jobs lighten in color over time, not darken. So I am very curious about your pool.
    Hope my reply to this is "on topic".... My pool is about 11 years old and we've been here only 4 years. I saw an estimate prepared for a re-plaster when it was only 3 years old that was never done so it may be that the original owners were unhappy with the original job. I do not expect that the pool was kept in good balance in those years; the Mrs. managed the property alone for 3 years post-divorce and she knew nothing about the pool. I see a lot of the mottling that you mention in the original post as well as these streaks that look like something the Polaris could have left behind.

    I will try sanding, as described, to see what happens. I will get a photo of it for you.
    23,000 gallon in ground pool with rock waterfall and spillover spa, Aqualink control system, Polaris 380 cleaner, Purex Triton Clean&Clear Plus cartridge filter. Located in The Woodlands, Texas.

    Pool owner since Nov 2008, Trouble Free since April 2009. Happy to help when I can.

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    Re: Gray Mottling Plaster Discoloration

    Since you didn't see the pool when new, I am now wondering if the original gray color plaster was darker than it is now, since usually dark colored plaster gets lighter over time? And that the current dark streaks are actually the original color?
    So sanding a couple of small areas with 80 or 100 grit sandpaper should tell the story. And it will be interesting to see if the raised areas (or streaks) is removed by the sanding.

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    Re: Gray Mottling Plaster Discoloration

    Found this thread - my pool is two months old and has this problem which is getting worse each week. Plaster contractor has been out and agrees it will need to be torched and/or acid washed. Wondering if anyone has done either of these and if it worked?

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    Re: Gray Mottling Plaster Discoloration

    It is nice that the plastering contractor acknowledged the problem, some won't. Torching and a very light acid wash sometimes work, but I have to tell you that the gray discoloration (if that is indeed the problem) may return again in a few weeks. Tell him to not "over do" the acid washing. That will shorten the life of the plaster finish.

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