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Old 19-06-2018, 06:02   #1
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Dark spots in water tanks

Hi,

I am trying to refit of a Southern Cross 31. It has two builtin fiberglass water tanks. They both have dark spots (see attached photos). I tried to wipe them out and the dark spots get much lighter and sometime almost disappear but the don't go away 100%.

Any idea on what these dark spots could be or how to treat them?

I was considering replacing these water tanks but that sounds like a big and expensive job

Thanks!!
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Old 19-06-2018, 06:26   #2
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Re: Dark spots in water tanks

Had a similar problem earlier this year. They don't wipe off but using a green scouring pad and some kitchen cleaner plus finger nail seemed to work well and removed them. Lumps were to be hard chalky like limescale but dark, mold? This was followed by rinsing and bleaching the tank just to be sure as I do every year.
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Old 19-06-2018, 07:21   #3
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Re: Dark spots in water tanks

Thanks Pete I will to wipe harder but they don't seem to go completely away no matter how hard i wipe. I thought it was mold but then I started to think about a potential material failure, like the gelcoat/fiberglass is blistering or something
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Old 19-06-2018, 07:26   #4
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Re: Dark spots in water tanks

The problem may be bacteria under hard water scale. You might want to try Trac Ecological's PSR as a remedy which seems to work pretty well.

FWIW...
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Old 19-06-2018, 07:28   #5
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Re: Dark spots in water tanks

The spots appear to be some type of algae. This Is confirmed since rubbing removes the spots.

I would not sand the area.

Spray the spots with bleach (Clorox) and let the bleach stay in contact. Be careful not to breath the mist. Wear a basic cloth mask while you spray and then get out of the area while the mist settles.
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Old 19-06-2018, 07:35   #6
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Re: Dark spots in water tanks

Read about purifying a water tank / system in Peggie’s book.... The head mistress. I would start by adding bleach as per her recommendations into the tank and lines. I had these in my prior boats fiberglass tank. Some came out and others didn’t full. But I was sure the tank was decontaminated and no mold.

Good luck!
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Old 19-06-2018, 14:06   #7
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Re: Dark spots in water tanks

Thank you all for your ideas and suggestions. I am going to try with bleach and see what happens although I have the feeling that the dark spots are not going to go away....
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Old 19-06-2018, 14:48   #8
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Re: Dark spots in water tanks

https://www.homedepot.com/p/Everbilt...-6HD/205047060

Sorry, having trouble inserting a link/pic of the Everbilt dryer vent cleaner. Home Despot less than $20.
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Old 19-06-2018, 14:52   #9
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Re: Dark spots in water tanks

Have a new to me 32 year old boat with twin 180 gallon water tanks that looked just like that before this past weekend (see video). Believe the tanks are fiberglass with an internal epoxy coating. From the caulk I had to chisel through with mini-pry bars, my guess is the tanks have never been opened since new. Used a dryer vent cleanout tool (see pics) along with 1 quart of unscented bleach in 10-15 gallons of water. Now the inside of the tanks looks like a new white tshirt. Put in the water and bleach, inserted the cleanout tool in a drill and went to town running the brush back and forth for the length & width of the tank. Added/removed sections of the fiberglass rod brush extensions to get to differenct areas of the tank. Actually couldn't see the extremes of the tank (only 8" opening under a panel at the bottom of a hanging locker), but got out lots of grit, the water was clear afterwards and the tank lining looked like a pure white life is good tshirt.
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Old 19-06-2018, 17:27   #10
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Re: Dark spots in water tanks

I donít even want to know what the inside of my 30 yr old tank looks like.
Actually I looked thru the small level sensor hole a couple of years ago. It was clean, except for what looked like calcium deposits.
Itís an aluminum tank.
I donít think you get algae in a water tank, cause I thought algae took light to grow? Maybe itís a kind of bacteria that looks like algae?
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Old 19-06-2018, 18:59   #11
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Re: Dark spots in water tanks

I filled tank full to brim, and added a gallon and a half of bleach ! Let sit for 2 days! Then did a part pump out, not much ! Then used elcheepo bristle brushes from cheapo depot to get at the the baffle walls, and sides as far as I could reach. Then I refilled to top and put a small air pump line into tank to agitate for a couple of days. The crud that loosened up after initial cleaning was amazing!
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Old 20-06-2018, 04:30   #12
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Re: Dark spots in water tanks

Update! I went to the boat yesterday and applied a little bit of bleach, let it sit for few minutes and then scrub it. Well... The spots seem to go away but once the spots went away and the surface was pretty clean I realized that there seem to be blisters all over the internal walls of the tank. You gotta play with the light to see them and I would say the biggest one is about the size of a dime. I took photos but it's hard to see the blisters in the photos.

Are these blisters/bumps normal? Do they need repair?
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Old 20-06-2018, 06:31   #13
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Re: Dark spots in water tanks

try to pop 1 or more open, then see if there like a pus inside! If there is you have osmotic blistering ! That needs to be fixed if you even think about drinking the water out of those tanks !
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Old 20-06-2018, 06:38   #14
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Re: Dark spots in water tanks

I think you could pop and fill the blisters as a repair. Then I'd coat the tank with food-grade epoxy to avoid new blisters and ensure safe drinking water.
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Old 20-06-2018, 07:00   #15
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Re: Dark spots in water tanks

The problem with popping and filling the blisters is that there are LOT of them and some are very difficult to reach...
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