Dry acid made black stains in plaster. Can they be removed?

[nelson411]

Hi everyone,
Welcome to the swim season! We have black stains in our grey pool plaster from adding dry acid that wasn’t diluted enough. We did it when we were new inground pool owners. Our plaster was redone in 2015. Ever since then we’ve had a difficult time with high PH and high calcium hardness. We just drained our pool 50% to replace the water. So we figured we could finally do something about the dark black stains.
Last pool test:
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Home
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Build Type: Plaster
Volume: 12500 gallons
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Latest Test Result Summary:
FC: 1.5 (4 days ago)
CC: 0.0 (4 days ago)
pH: 7.2 (4 days ago)
TA: 80 (4 days ago)
CH: 475 (4 days ago)
CYA: 15 (4 days ago)
BORATES: 0 (4 days ago)
TEMPERATURE: 84° (4 days ago)
==============================
Any help would be appreciated. I’ll post pictures soon.

Thanks

Liz

 

[blakeusa]

Have you tried brushing the area with a stainless steel brush.

 

[nelson411]

Have you tried brushing the area with a stainless steel brush.

No we haven’t. We’ve used a nylon brush before. I’ll give that a try. Thanks!

 

[Rollercoastr]

I wonder if it should be classified more as "damage" than a stain...?

Have you made the switch from dry to liquid acid?