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Old 30-09-2014, 08:57   #1
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Blackened bottom paint around zinc and through hull

Surveyed a sailboat yesterday. All the underwater metal is bonded together and then to a zinc plate outside the hull. The blue bottom paint was blackened around the zinc and the closest through hull. The surveyor recommended removing the zinc from the bonding circuit as it was "too much". No de-metalization on any of the underwater metal and zincs were eroding normally. Any thoughts on this?
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Old 30-09-2014, 21:13   #2
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Re: Blackened bottom paint around zinc and through hull

I suppose this really should be in the electrical section. Anyone ever seen something like this?
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Old 30-09-2014, 21:31   #3
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Re: Blackened bottom paint around zinc and through hull

Yup! I have.
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Old 30-09-2014, 21:53   #4
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Re: Blackened bottom paint around zinc and through hull

Just a few thoughts…………….

Could it be the discharge from the skin fitting, and the position of the zinc is coincidental??
What does the skin fitting discharge?

Is the blackening on top of the paint (a film?) or is it the paint actually changing?

Has there been any high voltage leaks to "burn" the paint via the skin fitting and tracking to the zinc?
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Old 01-10-2014, 05:43   #5
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Re: Blackened bottom paint around zinc and through hull

On mine it appears that the paint is changing and the thru hull is an intake.
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Old 01-10-2014, 10:10   #6
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Re: Blackened bottom paint around zinc and through hull

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On mine it appears that the paint is changing and the thru hull is an intake.
What you are experiencing is "burnback" and it is caused by the anti fouling paint reacting with an improperly primed metal fitting. Relatively common phenomenon.
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Old 01-10-2014, 10:21   #7
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Re: Blackened bottom paint around zinc and through hull

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Yup! I have.

This appears to be result of something being discharged from the thru-hull (typically soap or detergent) and coating the paint, IMHO. Happens a lot with galley sink drain thru-hulls.
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Old 01-10-2014, 17:50   #8
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Re: Blackened bottom paint around zinc and through hull

Thanks fstbttms, you've seen a lot of stuff at eyelevel. I value your input, in more threads than this.
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Old 01-10-2014, 18:01   #9
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Re: Blackened bottom paint around zinc and through hull

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What you are experiencing is "burnback" and it is caused by the anti fouling paint reacting with an improperly primed metal fitting. Relatively common phenomenon.
The zinc plate is set about 1/4" out from the hull and you don't want to prime that, so I guess just live with it? Is this a problem I should even worry about?

Thanks for the help!
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Old 01-10-2014, 19:03   #10
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Re: Blackened bottom paint around zinc and through hull

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The zinc plate is set about 1/4" out from the hull and you don't want to prime that, so I guess just live with it? Is this a problem I should even worry about?

Thanks for the help!
What is the anode protecting?
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Old 01-10-2014, 19:41   #11
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Re: Blackened bottom paint around zinc and through hull

It's bonded to all the thru hulls and underwater metal.
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Old 01-10-2014, 21:27   #12
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Re: Blackened bottom paint around zinc and through hull

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It's bonded to all the thru hulls and underwater metal.
Well, if you are married to having it on there, I guess you get to live with having it ruin your paint in that spot. Personally, I think bonding everything is a bad idea. I'd take it off, cut all the wires and remove the bolt(s) at the next haulout.
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Old 01-10-2014, 21:32   #13
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Re: Blackened bottom paint around zinc and through hull

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Well, if you are married to having it on there, I guess you get to live with having it ruin your paint in that spot. Personally, I think bonding everything is a bad idea. I'd take it off, cut all the wires and remove the bolt(s) at the next haulout.
That's what the surveyor said, so thanks for confirming.
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