White patches on grey plaster

[AndyfromTucson]

Does anyone have any theories what might be causing this discoloration of our gray plaster? It's been growing for years, and seemed to accelerate in the past year or two. I am wondering if replastering is the only option, or if maybe the "zero alkalinity acid treatment" might help.

Here are my pool's specs:

Volume: 20,000
Surface: Plaster
Cartridge filter
CircuPool RJ45 SWCG
S200 Maytronics cleaning robot
Boric acid: 50 lbs
Year built: 2010
Last refill: Jan 2018

pH: 7.8
CL: 3.5
TA: 70
CYA: 60
CH: 1,800+

I know my CH is crazy high and I shouldn't let it get that high. My fill water is pretty high CH, and being in Arizona we get a lot of evaporation, so, rightly or wrongly, I have been nursing the pool along with crazy high CH in lieu of refilling every year (it took less than 3 years for CH to get to 1,800). Adding 50lbs of Boric acid helped reduce scaling on my SWG. Also, during the hot months I've been able to keep SWG scaling manageable by keeping the pH low (7.2-7.6) and running the SWG when the water is coolest.

The only other possibly relevant factor is that for the first 7 years or so I didn't really monitor or adjust alkalinity. That said, the discoloration seemed to get worse after I started maintaining TA around 50-70, though obviously that could be coincidental.

Any ideas or insights would be greatly appreciated!

 

[JoyfulNoise]

My first suggestion would be … calcium scale. Your CH is way too high and no manner of scale control is going to help. Calcium scale is not always white but can very often be grey, brown and bluish depending on what impurities it picks up. It’s hard to tell from outside the water but you could buy a Jacks Magic stain ID kit to see what it tells you.

As for the future, you need a water softener hooked up to your house and a tap run to your auto fill plumbing or a spigot near the pool. I live in Tucson and you simply cannot operate a pool here using municipal water or even well water …