AluminumAlloyBoats.com

I have a 28' boat from a well-known builder in Oregon which I had built in 2017. Unfortunately, I chose to have it mostly painted. At the time of building, I asked about protecting stainless fittings with something like Tef-gel, but was told that their method was to use silicone on each fastener. Of course, this didn't work, and I have corrosion under the paint at just about every location something was mounted (plus some other places). I'd like to clean things up and was considering prepping and painting all of the little corrosion spots. But it recently occurred to me that I could try masking and scoring (maybe squares or circles) around each fitting, and then sanding to bare aluminum and call it done. Any other suggestions on how to clean this up?

Ethan
 
 

Ethan,
the pictures seem to indicate your builder's paint job didn't include an etch and chromic acid conversion of the surface to chromium oxide- accomplished with chemicals like Allodyne on wet rinsed acid etched 5000 and 6000 series aluminum?  The mill scale seems intact in the images, but the coloration may not be mill scale- can't tell for sure?

I see the correlation of SOME of the corrosion sites (wrinkled or lifted paint areas) but not all, to fasteners cutting into the paint job.  My implication from these images is the hull wasn't etched to remove mill scale before painting & there doesn't appear to be any oxide conversion to which a primer could adhere?? Again, I may be mistaken but I just don't see any evidence of a brownish conversion layer?

So my conclusion is the mill scale was left on, retaining is contents of water vapor which could get de-aerated and become acidic and then attack the metal directly as there is no oxide defense of the parent metal when the mill scale is left on.  The areas of lifted paint not associated with fasteners' galvanic corrosion; lead me to this impression.

If you don't strip the paint, etch and either convert the aluminum oxide to chromium oxide in order to repaint??? (lots of time and money!!) OR leave the hull bare etched metal;  then the 'cancerous' paint job will eventually leave the entire hull with the same corrosion pattern shown in the (now) small areas.

Just my view from past experiences with these visual symptoms.

good luck,
Cheers,
Kevin Morin
Kenai, AK

 

Thank you for the insight.

In unpainted areas of the boat (bilges, etc) it is clear to me that the mill scale was not removed.

At the time of build, I was told that the entire boat (at least the area to be painted) was sanded and etched. An epoxy primer was used, followed by a BASF Diamont paint. In the photos I sent, it looks as if the paint adhered well to the primer, but the primer has lifted from the aluminum. When I chip and scrape those areas, there's a bit of white powder corrosion which is released.