How to I approach this stain?

[Summer Day]

Hi there,

New to the forums, and a bit confused about how to approach a single iron (I think!) stain in the deep end of my saltwater pool.
The pool was built in the mid 1980s, used a handful of times then boarded over for decades. We bought the house in 2017, lifted off the boards, pressure washed, installed new equipment, and re-commissioned.
After a year, we emptied the pool again while we re-landscaped, including installing new coping and paving, then re-commissioned in August 2018.

Looking back at photos from when we originally cleaned and re-filled the pool, it looks like there was a mark in the same location (circled in pink in attached image) after pressure washing, but no-where near as significant.

The pool school etc talk about ascorbic acid, and using some agent to collect the iron sediment afterwards, but if I just have a single stain, can I apply a stocking of vitamin C tablets just over the single place? Will I need to add anything afterwards?

I really appreciate anyone's help. I'm new to this

 

[Dirk]

Hi, and welcome to TFP! Are you saying a stain developed twice, in the same spot, before and after the finish was cleaned? Just clarifying for our experts to give you a good answer. If that's the case, sounds a bit like the rebar in that spot is too close to the surface of the finish... How did you determine it's an iron stain? And in the first pic, are you referring to that black-brown spot to the left of the drain? I'm not really seeing anything in the second pic...

 

[Summer Day]

Thanks for the reply. The surface wasn't replaced as such, we just cleaned it with a high pressure water cleaner. I think it might be in the same place, but i'm not completely sure. If it is rebar poking through, do I have any options?

 

[Dirk]

Yes, but you probably won't like them. Digging out the rebar, or otherwise isolating it, and then patching the finish, which will never match. Though perhaps others here might have some better workaround. Cleaning the stain regularly would likely be a better solution, if in fact you can clean it.

Did you determine it is an iron stain?

 

[Summer Day]

No, I haven't done anything - I was scared off by the mnetion of iron precipitates in the water!
I wanted to make sure that I wasn't going to hurt anything by putting a stocking of vitamin C tablets in the pool.

 

[Dirk]

See if this article answers your questions:

Ascorbic Acid Treatment - Further Reading

www.troublefreepool.com

 

[mknauss]

Putting vitamin c tablets on it, if it is iron, will remove it quickly. The only effect to your water chemistry will be a reduction in FC, albeit small if only a few tablets are used.

You can set a magnet on it and see if it sticks. That would imply a rebar near the surface of the plaster.

 

[ajw22]

See if a magnet sticks to the stain area.

That looks like rusting rebar close to the surface and the rust leaching up into the plaster. You can keep on cleaning the area and it will return again. It will not go away until you cut out the rusting rebar, fill the hole with hydraulic cement, and patch the plaster or replaster.

 

[Summer Day]

Well the vitamin C tablets certainly worked, although not as much as I would have expected given the number I put on there. I am trying to convince a child to swim down there with a magnet, but no luck as yet!
Is a full ascorbic acid treatment likely to do a better job than settling vitamin c tablets on the stain itself?

 

[mknauss]

It might, but a full AA treatment is pretty involved. You have to let the FC go to zero, use a non-copper algaecide, treat the pool water (10 lbs of AA per 10000 gallons of water, maybe more), then slowly raise the FC. Since you are only dealing with a small spot, I would just use more tablets. Put them in a thin sock to set them on the stain. It will reoccur most likely.

 

[Summer Day]

Thank you! I will persist with with Vitamin C tablets then.
There are a number of reasons I can forsee needing to resurface the pool in future, so if it comes to that I will make sure they treat the exposed rebar at the same time.