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Thread: Stains On Gunite Pool

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    Join Date
    Mar 2015
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    Stains On Gunite Pool

    I just had my pool resurfaced in 2015 and in the last year it's developed a couple of stains that I'd like to see if I can get any pointers on how to remove. One type of stain is from pucks that were placed directly in the pool until they dissolved. The other seems to be... dirt? They're darkened areas primarily in the deep end where palm nuts drop, debris tends to collect, and the pool cleaner spends most of its time. However, these areas are completely unaffected by vigorous brushing, the cleaner, etc. so I'm not certain they're dirt. Nothing looks too bad, but the pool's surface is only a year old, and I don't like to think what it would be like if this pace keeps up for another 10 years. Any suggestions?
    15k gallon (?) kidney-shaped inground concrete (paint) pool & spa combo undergoing resurfacing. 6 floor jets, 2 side returns around steps, 1 skimmer also near steps. Spa has 4 jets. 1 hp Sta-Rite Supermax, 1.5 piping, Pentair CC100 cartridge filter (holds up to CC200), ComfortZone gas heater - old and broken, Master Pool return distributor (not sure what to call it). Pool Before Resurface

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    Re: Stains on Gunite Pool

    This is the puck stain
    IMG_20160527_184512354[1].jpg

    and here is the "dirty stained" bottom of the pool
    IMG_20160527_184445324[1].jpg
    15k gallon (?) kidney-shaped inground concrete (paint) pool & spa combo undergoing resurfacing. 6 floor jets, 2 side returns around steps, 1 skimmer also near steps. Spa has 4 jets. 1 hp Sta-Rite Supermax, 1.5 piping, Pentair CC100 cartridge filter (holds up to CC200), ComfortZone gas heater - old and broken, Master Pool return distributor (not sure what to call it). Pool Before Resurface

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    Richard320's Avatar
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    Re: Stains On Gunite Pool

    The puck stain is there for keeps. Trichlor is very acidic and it etched the concrete. I don't know if you've ever seen a concrete floor where someone has set a leaking car battery or if you've ever set a jug of pool acid down somewhere and left a ring, but it's the same.

    The dark stains could be organic from tannins leaching out or metals or even something embedded in calcium scale. Can you dive down and feel it? Is it rough, or just dark? Rough and raised means Calcium scale. If smooth, set some cheap Vitamin C tablets on the stain for a few minutes and see if they left a white spot where they were. I mashed a bunch up and them them in a knee-high nylon with some smooth river rocks and pushed it in place with the pool brush. If there are clean spots, it's metals. Last test is -- believe it or not -- set a trichlor puck on for about five minutes. You've seen what happens when it goes too long. If it fades some, the stains are organic and elevated FC levels and time will fade them. Do the Vitamin C and the trichlor in that order, because the trichlor will also lift metals and may give you a false positive for organics.
    16K freeform gunite with spa; Pentair 4000 DE filter; Century Whisperflow 1 HP; Pentair Minimax heater.
    Troublefree does not mean Maintenancefree. It's like brushing your teeth: You can spend a couple minutes a day and pennies a week or go to the dentist once a year and spend several thousand dollars.
    A pool is like a pet - you have to feed it every day, even the days you don't want to play with it!

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    Re: Stains On Gunite Pool

    Quote Originally Posted by Richard320 View Post
    The puck stain is there for keeps. Trichlor is very acidic and it etched the concrete. I don't know if you've ever seen a concrete floor where someone has set a leaking car battery or if you've ever set a jug of pool acid down somewhere and left a ring, but it's the same.

    The dark stains could be organic from tannins leaching out or metals or even something embedded in calcium scale. Can you dive down and feel it? Is it rough, or just dark? Rough and raised means Calcium scale. If smooth, set some cheap Vitamin C tablets on the stain for a few minutes and see if they left a white spot where they were. I mashed a bunch up and them them in a knee-high nylon with some smooth river rocks and pushed it in place with the pool brush. If there are clean spots, it's metals. Last test is -- believe it or not -- set a trichlor puck on for about five minutes. You've seen what happens when it goes too long. If it fades some, the stains are organic and elevated FC levels and time will fade them. Do the Vitamin C and the trichlor in that order, because the trichlor will also lift metals and may give you a false positive for organics.
    Thanks for the tips! I'll try them out soon. The floor is as smooth as the rest of the pool, you can't feel the dark spots. If it is metals, what do I do? More vitamin C? For how long? If it's organic (wouldn't surprise me), how high does the FC need to be to clean it? I don't want to make the pool unusable for weeks dealing with this.

    Bummer about the pool tab stain I wish I had known that would be such an issue.
    15k gallon (?) kidney-shaped inground concrete (paint) pool & spa combo undergoing resurfacing. 6 floor jets, 2 side returns around steps, 1 skimmer also near steps. Spa has 4 jets. 1 hp Sta-Rite Supermax, 1.5 piping, Pentair CC100 cartridge filter (holds up to CC200), ComfortZone gas heater - old and broken, Master Pool return distributor (not sure what to call it). Pool Before Resurface

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    duraleigh's Avatar
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    Re: Stains On Gunite Pool

    I think Richard320 is spot on. Hard to be exact but I bet they turn out to be organic stains. I would elevate the chlorine to twice the suggested level (depends on your CYA) and simply keep it there. Brush the stains when you can but they will probably lighten/disappear very slowly over maybe a couple of weeks. You can swim ANY TIME in that condition so you will lose no use of the pool.
    Dave S.
    42k vinyl and concrete pool, 1.5hp pump, 140gpm filter
    TFTestkits , PoolMath , Pool School

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