Gunite interor of pool has black patches

Hammmerhead

Member
Mar 18, 2019
10
Arkansas
Im new to a Gunite salt water pool. We always had a fresh water liner pool. When we bought the home there were black areas of the gunite that simply wouldn't come clean even with a stainless bristle brush. I have been told its from the PH being too high. I don't know if there is a process to fix this in a faster fashion. We did bring the PH down and it seemed to slowly get better, but to be honest, not many people know anything about gunite and saltwater in my area so everyone is just throwing guesses at a solution.

Has anyone dealt with this before?

Thanks
 

JamesW

TFP Expert
Mar 2, 2011
16,341
The gunite is the main structure of the pool shell. The part that you can see is the plaster, which is a thin layer on top of the gunite.

What color is the plaster?

Do you have pictures?
 
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Jimrahbe

Mod Squad
TFP Expert
LifeTime Supporter
Jul 7, 2014
14,193
Bedford, TX
hh,

Thanks for the address correction... :thumleft:

The fact that your pool is saltwater should not really have anything to do with the problem. A saltwater pool is just the same as a chlorine pool only it makes its own chlorine.

Pictures of the problem would help as James has requested..

Do you have plain white plaster, colored plaster or some type of pebble plaster??

When you brush it does it come off at all or does it do nothing?

Thanks,

Jim R.
 

Hammmerhead

Member
Mar 18, 2019
10
Arkansas
Thanks for the replies. It does not come off period, even with a stainless scrubber. Vitamin C tabs, dont do anything much to speak of either.
When we opened the pool last year, it was worse and the PH was extremely high. We brought it down over time, and I think the black started receding, but it still had a good bit where it was negligible without examining the photos. Im looking forward in taking the cover off soon and see how it progressed over winter.

Thanks again









 

JamesW

TFP Expert
Mar 2, 2011
16,341
I'm thinking that it might be really bad copper stains.

Can you use a knife or wood chisel to scrape off a spot to see if the color is just on the surface or if it goes all the way through?

You should only need to scrape a millimeter or so.

Do you know if any copper algaecide was used?

Does the pool have an ionizer or heater?
 

Hammmerhead

Member
Mar 18, 2019
10
Arkansas
Scaping with a screw driver does get rid of a lot of it, but the oxidation is very deep. I have found tons of pool additives the old owners should never have needed IMO. I used a metal removal solution, but don't know if it had any benefits. The pool has a Heater/Chiller noted in the pics. I wonder what would be the best treatment for oxidized copper? Im sure some shock and algicides were used over the years without knowing the future result
 

Hammmerhead

Member
Mar 18, 2019
10
Arkansas

Thanks Jim. Im afraid the pool has areas that are beyond just staining and its a heavy oxidation.

This guy seems to nail my problems as well.

1.0 out of 5 starsDid not work for me
April 24, 2014
Verified Purchase
"I have a salt water pool that has black stains in the plaster, these are not algae, but some metal impurity that was either in the salt or corrosion of pipes of the heater. The kit failed to identify the cause, I used the Magenta Stuff with no success. The only thing that worked was under water acid bath, I used more than 6 gallons so far."



I will continue to add acid if it is still there when I uncover, unless someone here has seen it this bad in other pools and has info.

I have combed everywhere on the web and just haven't seen anything this bad.
 

JamesW

TFP Expert
Mar 2, 2011
16,341
I think that it's probably copper. If it is, it won't come off easily. A no-drain acid wash or sulfamic acid (Jack's #2) might help, but it might not.

If you can get it off, the copper would still be in the water. So, you would need to drain and refill to get rid of the copper.

Draining the pool can risk having it pop out of the ground if the ground water is high.

A no-drain acid wash should not be done with the system on line because it can damage the system, especially anything metal.

Another option is to drain and acid wash, but that would remove a lot of plaster and make the plaster very rough.

When you scratch the surface, is it white or blue underneath?
 

Hammmerhead

Member
Mar 18, 2019
10
Arkansas
HH,

I was not telling you to use the treatment, only to use the test kit to confirm what the stains are..

Jim R.
Oh no. I appreciate your input. I checked out Jacks late last fall and I will be ordering the test kit unless a miracle happened over winter while it was covered.





I think that it's probably copper. If it is, it won't come off easily. A no-drain acid wash or sulfamic acid (Jack's #2) might help, but it might not.

If you can get it off, the copper would still be in the water. So, you would need to drain and refill to get rid of the copper.

Draining the pool can risk having it pop out of the ground if the ground water is high.

A no-drain acid wash should not be done with the system on line because it can damage the system, especially anything metal.

Another option is to drain and acid wash, but that would remove a lot of plaster and make the plaster very rough.

When you scratch the surface, is it white or blue underneath?

Its more like a Grey color if I remember correctly.

I used Vit C tabs in a sock and they would clean the calcium off the splash tiles and would lighten up the general areas of the plaster, but they wouldn't do anything for the dark areas and I even left tabs just laying on the steps to dissolve. The dark patches are very smooth and slick feeling, but they are there and seem to be thick.

I'm thinking the pool help did this over the course of a decade from algicides, shocks and winterizing products. When I started the pool up last spring the PH was deep into the 8's and needed a good bit of salt. I think I added about 6 gallons of Acid just to break it. I'm curious to see it in a few weeks and what has changed if anything.