Is it Cobalt spotting, help needed?

[antoniosp]

So over the last week I noticed lots of small spots on bottom of pool that look like rust. I did the Vitamin C test (used tablet dissolved above stain and several in a sock) but stains were not removed. I used a small sponge and scrubbing them got stain out, but only on smooth part of fiberglass pool not the textured areas on bottom.
Reading around the site, I came to conclusion it may be cobalt spotting especially since my Calcium is really low. I took my water to local pool store and here are my water chemistry numbers:
FC 3
TC 3
pH 7.8
TA 85
CH 40
CYA 80
No iron or copper detected.

I will try and take some underwater pictures later today, but wanted to get people's opinion, before I go buy and add calcium chloride dihydrate to raise it from 40 to around 250. Is that the right first step? Will this make these stains go away or just prevent from getting more?

 

[linen]

Re: Is it Cobalt spotting?

Post some pics. My understanding is cobalt stains should be removable with ascorbic acid (vit C). Adding calcium will not remove the staining, but it will help prevent future staining (if it is truly cobalt which it does not seem to be since VitC did nothing).

By the way with a cya of 80 and an swg, you should never let your FC drop below 4 ppm.

 

[antoniosp]

Re: Is it Cobalt spotting?

Pool store chemist suggested I lower PH to 7.6 by adding muriatic acid and then add baking soda about 10lbs to bring raise Alkalinity to about 120. Then address hardness being low by adding Calcium Chloride to stabilize things and prevent further staining, before we move to address removing the stains. He also indicated the FC at 3.0 is fine. thoughts?

 

[linen]

Re: Is it Cobalt spotting?

Pool store chemist suggested I lower PH to 7.6 by adding muriatic acid and then add baking soda about 10lbs to bring raise Alkalinity to about 120.

Lowering the ph is fine, but do not raise the TA...tfp recommended levels are 60-80 ppm for a swg.

He also indicated the FC at 3.0 is fine. thoughts?

Our recommendation is 4 ppm minimum for 80 ppm cya with a swg. One thing I can tell you, following pool store advice and tfp methods generally is a recipe for disaster.

What are you using for a test kit?

Then address hardness being low by adding Calcium Chloride to stabilize things and prevent further staining, before we move to address removing the stains.

I think you can do it either way. How long did you let the VitC sit on the stain? If vitamin C does not fix it, then you may not have cobalt and you may not need to raise the calcium, though raising it to about 220 ppm shouldn't hurt anything.

 

[antoniosp]

Re: Is it Cobalt spotting?

yeah that's what I thought about following their advise, will stick to TFP suggestions.
I am using the Pentair test kit tubes and solution, similar to the TF100 kit. Also numbers I posted are from what the pool store told me when I took water to them and watched them while they were doing it.
I let the VitC sit on it for at least 2-3 minutes if not more. I did both the tablet and vitamin C in sock test, none worked.
So pool builder suggested the Purple stuff bottle and pool store suggested Oxalic Acid bag to remove the stains. Suggestions on how to go about it?

 

[antoniosp]

So I tried the Oxalic acid bag and it worked on couple of stains they are removed overnight. I am wondering why the vitamin C test didn't work, I am thinking of trying it again in case I didn't leave them on stain long enough (about 30 minutes for the vitamin C test) and see if that works.

 

[antoniosp]

So i tried the Vitamin C test again in a sock and left it over the stain for a couple of hours but it didn't work. I guess will try the Oxalic Acid then since during the test with the bag it worked.