Cobalt stains in fiberglass pools

Melt In The Sun

TFP Expert
LifeTime Supporter
Oct 29, 2009
Tucson, AZ
My curiosity has been piqued by plucky71's thread here. Low CH in a fiberglass pool tends to lead to cobalt staining. Why is this? I presume it's not from dissolved cobalt in the water, or vinyl pools with low CH would suffer the same problem.

Is cobalt used in the pigments in blue fiberglass pools? If this were the case, I would think that people with white fiberglass pools such as plucky71, whoozer, or waterbear (so I hear; he was before my time) wouldn't have anything to worry about, but it is still universally recommended to keep a middling CH level. I guess my question is just: what is the source of the cobalt that causes the stains?


TFP Expert
Platinum Supporter
LifeTime Supporter
May 7, 2007
Silver Spring, MD
My understanding is that cobalt is naturally occurring in water, just as iron is. It much more common in well water than municipal water, but sometimes occurs at significant levels even in municipal water, again just as iron does.

Different surfaces have different likelihoods of getting stains. Metal levels need to be relatively high to stain vinyl, in-between to stain plaster, and can be quite low and still stain fiberglass. Cobalt doesn't usually occur at high enough levels to stain plaster, but fiberglass will stain at lower levels, levels that are much more common.

chem geek

TFP Expert
LifeTime Supporter
Mar 28, 2007
San Rafael, CA USA
Some additional info on cobalt staining and its sources is here where they say that cobalt is present in the resin material and that high chlorine levels lead to the staining. They say that a higher was finish can prevent the staining. As for why a higher CH, or perhaps a higher saturation index, seems to prevent the staining, that may be due to protection (or slower dissolving) of the gel coat which can contain calcium carbonate.

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