Thru Hulls & Bottom Paint Sloughing

[jim]

I have a CD 25D whose bottom I've coated for the last 4 years with Petit's Ablative Aqua Clean. This year for the first time, the paint has totally sloughed off around all of through hulls in an approx. 6" radius down to the gel coat. This paint has also sloughed off badly in large areas away from the through hulls in the past. I have done nothing to change the boat's electrical system's grounds etc. I don't know what the cause is but I'm concerned about the bronze through hull's integrity. The prop zinc shows normal erosion. Any thoughts? I wonder about stray currents at my marina.

thanks for any thoughts and advice.

jim

CYLAN CD25D, #99 at Guilford Town Marina, Guilford, CT



james.jamieson@yale.edu

[John R.]

I have a CD 25D whose bottom I've coated for the last 4 years with Petit's Ablative Aqua Clean. This year for the first time, the paint has totally sloughed off around all of through hulls in an approx. 6" radius down to the gel coat. This paint has also sloughed off badly in large areas away from the through hulls in the past. I have done nothing to change the boat's electrical system's grounds etc. I don't know what the cause is but I'm concerned about the bronze through hull's integrity. The prop zinc shows normal erosion. Any thoughts? I wonder about stray currents at my marina.

You pose an interesting question. Without being able to look at the area in person it is difficult to determine by description alone. How much do you use the boat? One possibility is that the water turbulence around the thru-hull area is affecting the paints adherence. I've never used that particular ablative paint but I'm thinking the normally controlled wearing away of the paint film may be excessive in those areas.

I doubt that stray current is causing the problem but you state it's the first time in four years that this has happened. That makes one wonder. However, on the flip side you also say it has worn off prematurely in large areas away from the thru hulls. That makes me believe it has nothing to do with any stray current. It may be resulting from an application problem, lack of stirring before application, a wipe down with a non-recommended solvent before application, etc.,. You should rethink all the steps that were done during the last application and see if you can recall anything that may have contributed to the problem.

Obviously just clean your thru-hulls and do a visual inspection of them looking closely for signs of dezincification. The bronze should have the nice goldish tone throughout its surface.

I would make a call to Petit and also seek out other owners who use that paint and get their opinions or ask the different yards in your area if they have seen this problem with the paint in the past.

If Aqua Clean is a water based paint then that would tell me immediately that there is a strong likelihood there is a possible formulation problem with that paint and my next step would definitely be that loud phone call to Petit.

[Bruce Barber]

Jim,

I've had this problem in the past, and it happened at every piece of bonded bronze including the groundplate. I talked about it with a couple of Annapolis-area experts in marine electrical systems. The one who claimed some firsthand experience said it was not an indication of corrosion, stray current, or of a marina AC system problem. The antifouling paint is conductive due to its copper content, although the paint companies "do not like to admit it." He's seen it frequently and said it's nothing to worry about, except that the paint is obviously not effective there after the copper goes (I guess it goes because copper is anodic with respect to bronze.) In my case, some paint remained but there was intense fouling, indicating no copper. His suggestion was to isolate the paint from the bronze fittings by leaving a 1/8" gap or so unpainted. I've since done that at the groundplate and it's worked. Noticing that the problem did not exist at thruhulls with unbonded seacocks (I had one), I also tried unbonding others to see what would happen, and found that that is also a solution. But of course you lose the benefits of bonding (if you are of that school of thought.) I'm not clear on why the bonding triggers the paint loss, or why some boats (most, it seems) don't have the problem. By the way, the paints I was using when this occurred were Interlux Aquarius (water-based) in 1997 and Micron CSC Extra in 1998. I never noticed any underwater metal corrosion, so (no matter what the mechanism) it doesn't appear to be a safety issue.

Bruce Barber
25D Nancy Dawson



bruce.barber@att.net