Alum OB Bracket - Glass - Copper Paint

Dear Group Wisdom,

I have a 22.8 Wellcraft Sportsman with a retrofitted Aluminum engine bracket and 200hp Envinrude. IO to outboard conversion worked very well. Boat lives in the water. Entire motor comes out of the water all the way. The only metal in the water is stainless trim tabs but they are isolated from any other metal. Recently had the bottom sandblasted. Will now put on barrier coat and ablative copper paint on the hull. The sandblaster mistakenly also blasted the aluminum bracket. Now what to do with it. I'm considering getting ride of the zinc and drain plug on the bracket and wrap the below water line area in epoxy and glass and paint it with copper antifouling that works so much better than the off the shelf antifouling for aluminum. I would apply the first coat of epoxy per Gougeon Bros. instructions: Wet sand the aluminum with the epoxy to get fresh aluminum exposed void of Oxygen.

I know Aluminum and Copper are a no-no, however, with a nice substanial layer of resin and glass between them, seems like it would be OK. Anything wrong with this idea???

James Mills

 

Depends how often you use the boat, how long it sits in the water. Do you really need the Glass? I would use an etch primer then several coats of epoxy and stop there. At least you can easily repaint if the coating is damaged. I think the idea of glasssing the aluminium will give nothing but problems.

 

I have been looking for the data sheets that I had for the thermal behaviour of fiberglass as opposed to Aluminium, and have been unable to find it, but they are vastly different, I think the Aluminum responds to temperature change about 4 times as much as the glass fibers do. And the aluminium expands in all directions, the glass won't (it's borosilicate fiber, isn't it?) I would advocate epoxy coatings though, and total encapsulation of an aluminum part (like a backing plate), but this doesn't seem right to me. Then again, if Gougeon Bros. told you it's Okay, who am I to say otherwise.
My experience with glassing aluminum externals onto fiberglass hulls has always turned out to be unsatisfactory.

 

After hashing it around in my head a bit, I think I'm inclined now to freshly sand the aluminum, acid prep it, and then put 4 coats of a barrier coat on it. Paint it with TriLux II. Perhaps TriLux II works better than the regualr TriLux which I have found works about half as effective as good copper bottom paint. Anybody have suggestions on a good bottom paint for aluminum, I'm all hears.

I read once about Gougeon Bros recommending the wet sand technique for getting expoxy adhesion to aluminum, however, just the other day I saw in one of their Epoxy Works mags adhesion test results for epoxy on aluminum. The acid primer came out slightly better over the wet sanding.

Also, a couple of years ago, I toured an aluminum yacht builder's plant in Bradenton. No longer there. This builder had a cool process of taking long sheets of alloy from Germany and folding them up orogami style into a hull. No or very few transverse welds. Also, he was very proud of a special sprayed on hard and thick coating for the aluminum. When I asked him, with the thick coating on the hull, could he use regualr copper paint now, he was taken aback by the suggestion. Of course he would never use copper paint on an aluminum hull.

Thanks for the replies

James Mills

 

Copper & Aluminium paint

The whole idea sounds bad to me. You are already familiar with the interactivity between copper and alu. Lesson no 1 in shipbuilding is keep copper/bronze/carbon/graphite far away from alu.
Even with a glass-resin layer in between remains the question how do you bond alu and glass? In any case, if you really must, do it with epoxy-resin on a desert dry and clean hull. If yo know how to work with epoxy resin, you are ok, if not, abandon the whole idea and look for something less expensive and more practical. There are enough undercoatings to avoid the use of copperpaint as an anti-fouling.
Do you want to have a splendid anti-fouling provided you take the boat after each seson out of the water.
Go to a veterinary products wholesaler and by about 500 ml broad-spectrum antibiotic (not for human use) mikx that with the volume nescessary to paint your underwatership (formula in sq. mtrs lxbx 075) and you are ready for the season.