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Thread: Black stain on tile grout

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    Black stain on tile grout

    Hoping someone can help. Fiberglass pool with SWG and waterline tile. Pool is about 3 years old. The side that is full shade has black stains starting to appear in the grout. I have tried chlorine in a spray bottle with a brush. No luck. I have tried soft scrub and a brush. No luck. Lastly I tried a stain remover granular product in a sock that you scrub against the tiles. No luck.

    The black strain will not rub off with my fingernail and almost seems to be "in" the grout.

    It exists below the waterline, at the waterline and above the water line.

    Any ideas on how to remove? Is it unsafe in any way?
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    BoDarville's Avatar
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    Re: Black stain on tile grout

    It may be black algae. It usually appears on the pool surface as isolated black spots, dark circular patterns, or blotchy black patches. Black algae typically afflicts plaster pool surfaces and tile grout since they are somewhat porous. It is virtually impossible on vinyl or fiberglass surfaces due to the smooth surface that prevent the roots from embedding themselves. Take a look at some sample pics here: Black Algae

    Since you stated that you cannot get it to come off with your fingernail, try using a butter knife, or similar dull implement, and gently scrape a small sample of the stain and then smudge it on plain white paper. If the smudge is a greenish color, then it’s black algae. Please perform that test and report back with the results.
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    Re: Black stain on tile grout

    Quote Originally Posted by BoDarville View Post
    It may be black algae. It usually appears on the pool surface as isolated black spots, dark circular patterns, or blotchy black patches. Black algae typically afflicts plaster pool surfaces and tile grout since they are somewhat porous. It is virtually impossible on vinyl or fiberglass surfaces due to the smooth surface that prevent the roots from embedding themselves. Take a look at some sample pics here: Black Algae

    Since you stated that you cannot get it to come off with your fingernail, try using a butter knife, or similar dull implement, and gently scrape a small sample of the stain and then smudge it on plain white paper. If the smudge is a greenish color, then itís black algae. Please perform that test and report back with the results.
    Thanks! Will do that first thing tomorrow morning and will report back.

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    Re: Black stain on tile grout

    Looks like black algae or mold. Is the grout silicone?

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    Re: Black stain on tile grout

    Not sure if it is silicone as the tile was on the pool when it arrived as the manufacturer installed the tile. Any way to tell? Is silicone not the right type of grout?

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    Re: Black stain on tile grout

    Quote Originally Posted by gjpe View Post
    Thanks! Will do that first thing tomorrow morning and will report back.
    Got a picture of it but not sure how well you will be able to see it. It does NOT appear to have a greenish color. Any thoughts? Thanks again for any help with this!
    Attached Images Attached Images

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    Re: Black stain on tile grout

    Looks like black mold.

    Silicone would be soft and spongy. It would feel like silicone bathroom caulk.

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    Re: Black stain on tile grout

    Quote Originally Posted by JamesW View Post
    Looks like black mold.

    Silicone would be soft and spongy. It would feel like silicone bathroom caulk.
    Based on that, I believe I have silicone. Now, any suggestions on how to best remove black mold from silicone?

  9. Back To Top    #9

    Re: Black stain on tile grout

    Bleach on a toothbrush should get it.

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