Does this look like cobalt staining?

[Terry]

First photo will give you an idea of the relative size of the black spots, second photo is macro and is better to see the actual spots. This is located on the 1st and 2nd steps on my fiberglass pool. The variation in color is from paint that was applied 11 years ago and has seen better days.

I read that cobalt is raised and can be scraped off, but I cannot tell if these are raised or not. When the pool was painted the contractor sprinkled some sand granules on the steps to provide some traction so the whole area is finely textured. I have not been able to scrape any off, but have not tried too hard.

I have several other spots in the pool, all on the walls, where I am pretty sure I have some cobalt staining, but these areas are bigger- just a little smaller than a dime, and there is only one or two in the area, not so many tiny dots.

I intend to raise my CH this weekend, shooting for around 300 based on my reading- unless someone has other advice. Does the calcium actually reverse (remove) cobalt staining or just prevent more from occurring?

 

[Guest]

That does look like cobalt spotting. Both Ascorbic acid and Oxalic acid are effective treatments.

 

[Terry]

Thank you! So, I take it then that the calcium will do nothing for the current spotting?

Should a vitamin c tablet placed on one of the spots remove it? I had some vitamin c tablets on what I thought were iron stains (from roofing nails) but it made no impact on the stains. I will try this evening on the cobalt spots and see what I find.

Thanks again for the help!

 

[Guest]

raising the Ch will help prevent their return. Vitamin c IS ascorbic acid. See if it works. If not your might need oxalic acid (such as United Chemical's Cobalt Cure).

 

[Terry]

Yea, I know vitamin c is ascorbic acid- but calling it an ascorbic acid tablet just didn’t seem quite right!

I rubbed some vitamin c tablets on the spots w/ mixed results. In one area I was able to get the black to fade to a light yellow/orange color, but in other areas it made no impact at all. Is this what typically happens when ascorbic acid is placed on a cobalt stain (the yellow/orange color?)

I suppose that since I wasn’t able to get all the areas to respond to the vitamin c then the oxalic acid is called for. I read some on the Cobalt Cure and it appears the directions call for large quantities of acid and reducing the TA to zero during the treatment. Using the pool calculator it appears I would need 3+ gallons of MA and my pH would end up some where around 5.8. This is a bit scary to say the least, that is, purposely taking my numbers that far off target!

Ultimately I am going to have to have the pool refinished. I have put it off mostly because I haven’t been able to determine what would be best; re-gelcoating, epoxy paint, etc. Would it be better to treat the cobalt staining before or is it a moot point if the pool will be drained, sanded, etc. I suppose what I’m asking is does the treatment actually do anything to ‘fix’ the spotting- something I would want to fix before having the pool refinished? Or, is there any potential harm to the pool in not treating?