Blackened bottom paint around zinc and through hull

[azsailor12]

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?

[azsailor12]

I suppose this really should be in the electrical section. Anyone ever seen something like this?

[Roy M]

Yup! I have.

[40 South]

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?

[azsailor12]

On mine it appears that the paint is changing and the thru hull is an intake.

[fstbttms]

Originally Posted by azsailor12

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.

[fstbttms]

Originally Posted by Roy M

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.

[Roy M]

Thanks fstbttms, you've seen a lot of stuff at eyelevel. I value your input, in more threads than this.

[azsailor12]

Originally Posted by fstbttms

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!

[fstbttms]

Originally Posted by azsailor12

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?

[azsailor12]

It's bonded to all the thru hulls and underwater metal.

[fstbttms]

Originally Posted by azsailor12

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.

[azsailor12]

Originally Posted by fstbttms

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.