Scum Line Removal Suggestions

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I have an inground, gunite pool (14 X 28) using a chlorine generator.

Last winter, I lowered the water level below the skimmers to prevent freezing of the skimmers. When it was time to open the pool back up, there was a scum line (for lack of a better description) at the level where I had lowered the water. I cannot get that scum line removed!

Any suggestions? I've bought and tried out one of those 'blocks' that supposedly remove any/all stains. It didn't do a thing! I've tried using a stiff-bristled brush; no help!

The pool is drained now in preparation for a replastering job and another winter is on the way. Is there something I can do to prevent this from happening again during Winter 2019? Any suggestions to remove these stains just in case it happens again this coming winter? (I'd like to try out any methods now, before the pool gets replastered).

Thanks,
Joe Murphy
Nags Head, North Carolina

 

[kimkats]

Muriatic acid? 50/50 mixture followed with a good brushing and rinsing?

Do you get that many freezes up there?

 

[sktn77a]

I's start with a more dilute muriatic acid solution (4:1 tops) and increase strength as necessary.