Black spot on Diamond Brite pool surface

bean9677

Member
Jun 23, 2019
11
New Orleans, Louisiana
I have a black stain on the wall of my pool, it is ~ 1.5' below the water line. I have showed it to the people at the local pool store and they had me treat it for black algae, but it did not take care of the issue. Please see attached pictures, can anyone ID that spot, if so would you know how to treat it.
 

Attachments

mknauss

Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
22,396
Laughlin, NV
Welcome to the forum!
As it is on the wall, our normal suggestion to put a trichlor puck on it for a few minutes is more difficult. Do you have any cal hypo or dichor? If so, put some in an old sock, and press that against it for a few minutes using a pair of tongs or something as the solution (if you use dichlor) will be acidic. See how it affects the spot.
I suggest you read ABC's of Pool Water Chemistry and consider reviewing the entire Pool School eBook.
 

JamesW

TFP Expert
Mar 2, 2011
16,340
It looks like something metal in the plaster. Maybe a wire tie for the rebar.

See if a strong magnet is attracted to the spot.

See if vitamin C makes any difference.
 

bean9677

Member
Jun 23, 2019
11
New Orleans, Louisiana
I taped a small, strong magnet to a wooden pencil and put the magnet on the black spot. The magnet and pencil were held in place under water on the spot in question. Does this mean it is rebar, tie wire or some other iron coming through the plaster from the backside? If so what can be done to repair it?
 

JamesW

TFP Expert
Mar 2, 2011
16,340
I suspect that it's probably a tie wire.

To fix it, you would need to cut out the plaster and metal so that the metal is not near the surface. Then, patch the spot.

It might come back if water bleeds through and causes the iron to rust again.

Try some vitamin C on the spot to see if it helps.
 

mknauss

Mod Squad
TFP Expert
Bronze Supporter
May 3, 2014
22,396
Laughlin, NV
@JamesW for the win -- I should realize he is nearly always right -----

It is an iron tie most likely. The fix will not be pretty. Call your pool builder. How long ago was the pool built?
 

bean9677

Member
Jun 23, 2019
11
New Orleans, Louisiana
The pool was installed 04/2018. I have spoke to the pool builder, they are suppose to send someone to look at it next week. I will try some vitamin C this afternoon. What should the vitamin C do to the spot?
 

JamesW

TFP Expert
Mar 2, 2011
16,340
If you zoom in on the picture, you can see 2 tiny spots that look like rust. I would suspect that the vitamin C will clean the rust and hopefully the black.
 

JamesW

TFP Expert
Mar 2, 2011
16,340
You can show them the pictures.

Showing that the magnet only sticks in that spot should prove that the metal is near the surface.

I would do the vitamin C test now to help confirm that it's iron.