Tiny Brown Stains

[matj6876]

All

Over the last week I have noticed some small rust brown stains on/in the plaster of my pool.
The plaster is dark, so the stains are not easy to see and the plaster is less than a year old, so is still changing slightly itself.
The stains are tiny, less than 1/4" in diameter, round and rust brown in color. They seem to be pretty stubborn and do not come off with brushing. At last count, this lunchtime, I found about 25 of them, spread fairly evenly across a 15000 gal pool.

My numbers are pretty good, I think:
FC 5.0
CC 0.5
TC 5.5
pH 7.7
TA 90
CH 360
CYA 70
Borates 0
Temp 80F
Salt 4000

Any idea what this might be?

Cheers

 

[JasonLion]

Try rubbing them gently with a sock that contains a couple of crushed vitamin C tablets and see if that does anything. If that removes them then they are probably iron stains.

 

[matj6876]

Try rubbing them gently with a sock that contains a couple of crushed vitamin C tablets and see if that does anything. If that removes them then they are probably iron stains.

OK - so that worked. So I have Iron? Don't know where it came from!
Do I just treat all the spots and hope they don't come back or do I have to pay throught the nose for a sequestering agent to slowly remove the iron via the filter?

 

[JasonLion]

There are two possibilities. There could be a fair bit of iron in the water, which is depositing as stains on the walls. If so you will need sequesterant. Or they might be rebar rusting through the paster where it is too thin. Which is more complex to fix. Two ways to tell the difference: have the water checked for iron, and see if the stains always appear in the same limited number of places.

 

[matj6876]

I finally got around to getting to the local pool store and asked them to run a metals test. No iron, no copper. :?

Now I'm worried that I have rebar rusting through my 6 month old plaster in my 6 month old pool!

Thing is, these stains are all over the place, not in one loacation. And they are small - I would have expected rebar to rust through with bigger stains.

As I know there is no iron in the water I'm going to get them all off with vit C tablets tomorrow but does anyone have any idea what else could be causing this? Does anyone have experience with rebar rusting through plaster and what it looks like?