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Thread: Darkening on floor of new plaster job

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    Darkening on floor of new plaster job

    Commercial hotel pool, 23,000 gallons, 22 years old. I'm assistant manager of the property.

    It's been about 10 weeks since we re-plastered our pool. No start-up help provided by the contractor other than "brush it twice daily for 2 weeks" - nothing about chemicals or special treatments. After 6 weeks, we noticed blotchy dark stains all over the floor of the pool. Talked to the contractor, he said if we drain it he would come up and acid wash it, blamed us for not using a metal sequestering agent and said it was probably iron and manganese and copper. We drained it, his guy acid washed it, ground down a few spots here and there, and then (on a day I wasn't personally here) reportedly used a torch to "bring the stains up" before acid washing one area.

    That area is still darker than the surrounding, never really looked like the stain had come up properly. We started using a metal sequestering agent about 12 hours after starting the re-fill.

    After reading on this site for the last few days, I understand that acid washing lowers the life expectancy of the plaster. After having our local pool store test the water, they found no metals but a higher level of total hardness than our calcium hardness test + tap water hardness (they measured that too for us) can account for.

    Testing today (Taylor test kit):
    pH 7.6 (added 8oz Aqua Guard pH Down to bring down a little)
    FC 8.0 (shocked it last night due to an oil slick showing up on the surface, letting it drop down naturally in the sun)
    CC .5 or less
    TA 100
    CH 160
    CYA < 30

    Please understand that I have to keep it within limits set by Washington State Department of Health and I can't close it for any measurable length of time....

    But what do I do?

  2. Back To Top    #2

    Re: Darkening on floor of new plaster job

    here's a pic:

    you can see the stain spread out from top left to lower right.

  3. Back To Top    #3

    Join Date
    Mar 2007
    San Rafael, CA USA

    Re: Darkening on floor of new plaster job

    Welcome to TFP!

    The acid wash may have had plaster dissolve into the water. That would increase CH as well as pH (and TA).

    Others here nor more about what to do for water balance after an acid wash so I'll let them chime in. I would assume you make some adjustments to have the saturation index be closer to 0, but I'm not sure.
    16,000 gallon outdoor in-ground 16'x32' plaster pool; Pentair Intelliflo VF pump; Pentair IntelliTouch i9+3s control system; Jandy CL-340 square foot cartridge filter
    12 Fafco solar panels; Purex Triton PowerMax 250 natural gas heater (200,000 BTU/hr output); automatic electric pool safety cover; 4-wheel pressure-side "The Pool Cleaner"

  4. Back To Top    #4

    Join Date
    Jul 2011
    Arizona & California

    Re: Darkening on floor of new plaster job

    Normally, a stain that deposits onto a new plaster surface can be easily removed by an acid wash or with sanding. It sounds like the acid wash did not work, so apparently the plaster itself is discolored and is not a stain. That may be why the contractor used a propane torch to try to whiten or lighten the plaster color. You need to question the contractor and ask him if the plaster has "gray mottling discoloration."

    For more information, read this post: gray-mottling-plaster-discoloration-t44424.html

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