Little brown rust spots on fiberglass pool...

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tfpusername

tfpusername

Well-known member
Apr 23, 2017
119
Las Vegas
A year ago I noticed some little brown spots (half a penny size) at the bottom corner of my deep end (assuming they are rust?). I have been keeping the chemical balance perfect these last several months (chlorine, ph, etc...), and haven't noticed any new spots, until today. There is one new spot in the shallow end, right in the middle of the pool!

Any idea what's causing these spots? The only thing that isn't perfect is my water hardness (it's a bit high 500ppm).

IMG_20171211_102946.jpg

Thanks for any advice!

*I have a full test kit if you want me to check anything
 
The easiest thing to check first is chemistry values.
Post up a full set of test results.

Do you follow the [FC/CYA][/FC/CYA]?

Typically brown spots equate to metal/iron objects in the pool. Or iron containing pellets from fertilizer. Anything like that around?

Take care.
 
Put some crushed up vitamin C tablets in a nylon and use your pool pole to drop it on the spot. If it disappears, then it's iron. If you have a trichlor puck or some granular chlorine (dichlor), you can do the same thing to test and see if it's organic. It should only need to be applied for no more than 5mins to see some results.

Fiberglass pools can suffer from cobalt-spotting but that usually looks like dark brown/black pimples all over the pool surface.
 
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Ok was able to snap a decent photo...

IMG_20171211_102946.jpg

The spots do kinda look like pimples now that you mention it. Most of them actually look like tiny circles (with white/clean spots in the middle).

I still went ahead and dropped in a skimmer sock filled with 6x 1000mg crushed vit C tablets. It's been sitting out there for about 15min, will go check it soon.

As for the Chemistry, lke I said it's all pretty solid - 500ppm Total Hardness, 3 TC, 3 FC, 7.5 PH, 90 TA, 35 CA
 
Being along the transition from floor to wall, looks like algae. Will be interesting to see what the vitamin C does. If it does not effect it, try a puck.

Do you brush often or use a in pool auto cleaner of any kind?

Take care.
 
Well the vit C did nothing.

I brush about once a week and I have a Dolphin robot with two scrubbers. Whatever these spots are, they are like burned into the fiberglass (no amount of scrubbing seems to put a dent in them). They are smooth to the touch though (not rough like you would expect rust to be).

I don't have any chlorine pucks (I was told not to use them cause they mess with the CYA?), so I will need to get some at the store. Although with as hard as I have scrubbed these spots, I am not confident a chlorine puck is gonna break them up.

What about that "cobalt-spotting" suggestion? How can we test for that?
 
Did a quick search on Cobalt Staining. Might be it - but - it typically only occurs when CH levels are very low. And yours is not.

There is a product to remove the stains. https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=3&cad=rja&uact=8&ved=0ahUKEwitkd-HgIPYAhVH9YMKHQs_B0oQFgg4MAI&url=http%3A%2F%2Fjacksmagic.com%2Fproduct%2Fstain-solution-1-the-iron-cobalt-spot-etching-stuff%2F&usg=AOvVaw2Ap7z0px4i6yZWRwMgcbEd

Good luck.

- - - Updated - - -

Any chance they could be copper? Any use of copper algaecides - heater with copper exchanger, etc?
 
I would suggest using the Jack's Magic Stain ID Kit before doing one of their stain treatments. It will give you an idea of what metal stain it might be.

I have used both the Iron, Cobalt (#1) and the Copper (#2) treatments. They work beautifully, but they can easily come back. I have found that to keep the staining at a minimum, I add their Blue Stuff weekly and I keep my pH on the low end (7.2-7.4).

If you decide to do one of their treatments, know that your CC will be off the chart for awhile. The last one I did, my CC did not drop for about 8 weeks. (Read this old thread: Rebalancing after Jack's Magic #2 )
 
This year I had some similar spotting in my fiberglass pool in exactly the same place and fashion. In my case, I thought it was because I got lazy and got in the habit of pushing (sweeping) debris towards the deep end until I would get around to vacuuming days later. I figured it was organic stains from bugs and such that I allowed to just sit there for too long. Still not sure, but it was my guess. From there, scrubbing lightened them slightly, but they remained for a few months until I continued to keep the FC elevated a bit and brushed/scrubbed frequently. The spots haven't caught my eye recently, so I guess that's a good thing in that they've faded enough to where I don't notice them anymore. While bugs may not be the culprit in your case, perhaps something else managed to settle in the deep end just long enough to be a nuisance and avoid the Dolphin. Interesting that it seems to be predominantly just in that location, nestled-in that bottom crease of the pool....same as mine. If you do try the Magic's Stain kit, I too will be curious to see what you get. If there's a down-side to TFP, it's that our water is so clear we can see every imperfection below. :hammer:
 
Not sure about copper (we have no heater)

Fun fact about this pool - It was drained for 8+ years, with a foot of brown mucky water sitting at that deep end corner. After draining it/scrubbing it there was only a slight darkened stain (no brown spots). The spots only appeared maybe a couple months after we had it filled/chem balanced.

And is this the kit I need?

https://www.amazon.com/Jacks-Magic-JMSTAND-Stand-Kit/dp/B003MY6QQI/ref=sr_1_2?ie=UTF8&qid=1513043893&sr=8-2&keywords=jacks+magic+stain+id+kit

Or are there any other safe tests I can try first?
 

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Try the trichlor puck before you buy the kit. You can buy one at any local pool store and they cost you a buck or two.
 
That is the correct kit.

Definitely try the trichlor first. The location of my stains were sporadic and not in any kind of specific location like in your pic. I know that cobalt is a byproduct of fiberglass, which can lead to that staining. Copper may be from fill water or other additives as we had no heater at that time.
 
Welp just let a chlorine puck sit on a cluster of spots for about 15min, then tried to scrub it with my pool brush. Not even a dent.

I left the puck on for another 15min, but still not even a slight change.

So we know chlorine won't budge it. We're pretty sure it's not rust (the crushed vit C had no impact, plus I would expect rust to be rough, but these spots are smooth).

Is it time to order the test kit linked above? Or is there some other test I can try first? I mean assuming it is cobalt, what is the treatment method? These spots feel like they are burned in hard, I can't imagine anything getting them off...
 
I would talk to the fiberglass installer and ask them. Because it did not react to chlorine or vitamin C, it is more likely that it is cobalt spotting. Cobalt napthenate is used as a resin curing accelerant when the fiberglass is manufactured - spun glass fibers are laid out in the form and then the hard polyester and polyethylene resin is poured over the fiber. The cobalt accelerant is added to the resin just prior to the pour to decrease the gel time of the resin. Typically cobalt accelerants are kept to below 2% in order to avoid premature gelling before the resin has had time to penetrate the fiberglass. When the fiberglass gel coat becomes compromised, pool water can react with the cobalt in the resin and causes it to oxidize. This is the black spot you would see as a spot deep in the resin and not raised above the surface.


I'm not sure there's any chemical spot treatments that can fix this. You could call up Jack's Magic and talk to their technical support division to see if they have any products for possible cobalt spotting. If that fails, then the spots might require mechanical grinding to remove them and then a patch repair of the FG surface. It's best to contact the manufacturer to get their opinion on it. But be warned, they will more than likely try to blame you and adverse water chemistry for the spots. I don't know what terms are in your warranty, but you might want to check your documentation on the pool shell to see what kinds of exclusions are present.

Good luck.
 
I would try the Jack’s Magic stain ID kit. It’s not expensive, and will lead you in the right direction for treating potential metal stains.

For mine, I add about 4 ounces of their Blue Stuff each week, and keep my pH in the low end of the proper range. This has kept mine at bay for the last year.
 
@Joyful unfortunately the pool was installed 20+ years ago, I wouldn't even know who to call. Unless somebody here on the forums has experience with these spots/fiberglass and can make a recommendation, I think I am out of luck.

I suppose the Jacks's Magic stain ID kit is worth a shot...
 
Ok, I thought it was a new installation. Can’t hurt to give the stain ID kit a try. If something in the kit does lighten the stain then perhaps it’s close enough to the surface that a chemical treatment might work. Let us know how it goes.
 
Hmm just noticed the stain kit says "water should be 65-70 degrees", I don't have a thermometer, but the lows here in Vegas are around 45, so does that mean I need to wait?

And what exactly is in this kit? It looks like just a powder/liquid? How am I supposed to apply it to spots at the bottom of the deep end?
 

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