Jun 10, 2016 0 comment

“I am having trouble keeping paint on the lower leg of my outboard motor. This is a small, aluminum, 15-horse Evinrude, which stays in Lake Ontario from May to October each year. The difficulty seems to be twofold, with lack of paint adhesion being one problem, and of course corrosion another. I have tried a lot of different things, and last year went through what I had thought was going to be a very good preparation. I sandblasted to bare metal, prepped the aluminum with a 2-step (probably chromium) etch, applied zinc chromate primer. All this was followed by the outboard manufacturer’s lacquer, and finally a TBT (tributyl tin) top coat for antifouling. All paints were applied from spray cans, and strictly according to the manufacturer’s recommendations. This held up well for a couple of months, but still failed. However, it worked much better than just painting over a wire-brushed surface. Interestingly, I also did the controls the same way at the same time, with the same coatings (except for the antifouling top coat), and the controls are fine after a year. Of course, they are not submerged for 6 months at a time. The primary purpose in this case is protection, not aesthetics. Can you make any recommendations? I had also considered stray electrolytic or galvanic currents as a factor. I have two sacrificial teardrop zincs bolted to the anticavitation plate of the motor. Since it is electric start, it is well-connected via 10 AWG wire to the 12V battery ground. The bronze thru-hulls and external lead keel ballast are also “bonded” into the 12V ground. The zincs are fastened to the motor by stainless steel bolts. I applied a light film of lithium grease to the area between the zincs and the aluminum of the anticavitation plate, which is unpainted. The zincs did show some oxidation after 1 season, but no massive erosion. It seems that corrosion is now only a small part of the problem. I do use a regular, automotive-grade battery charger occasionally, so I am sure that there is some AC leakage to ground during charging. I do keep the charging to a minimum, since the outboard is alternator-equipped. At this point, I am thinking that the coating (in my case, lacquer) is not impervious to continuous submersion. It seems that deeper down the lower unit, the worse the failure of the paint. Since the top area sloshes in and out of the water, it is not continuously submerged. In any event, the prep and paint, combined with the addition of the zinc anodes, had by far the best results. I am thinking that epoxy or polyester powdercoat would allow less water migration. I know that when preparing fiberglass hulls, the standard is to paint them with an epoxy water barrier before applying antifouling bottom paint. This reduces the occurrence of osmotic blisters in the polyester laminate. Also, when installing and removing the motor in the Spring and Fall (the motor is mounted in a “well” inside the lazarette), there is always some paint chipping off the edges of the prop and anticavitation plate that exposes bare aluminum. Hopefully powdercoating would resist the chipping a little better. I have also considered having the drive unit plated, but even nickel plating would probably require a copper base plating, which would react with the aluminum substrate underwater, essentially creating a self-destructing battery. Do you think epoxy powdercoat is the way to go? Maybe two coats are better than one.”

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“An epoxy powder coating would be the ideal paint for your application. A zinc-rich epoxy primer coating would be even better. Two coats would be better but the top coat should be polyester for its UV stability (of which epoxy has none). UV stability would resist chalking on sunlight exposed surfaces.

You will have to completely disassemble the entire lower unit of the outboard motor before powder coating. Cure temperatures of 350 to 400 degrees F will cook all bearings, seals, o-rings, etc. For this reason, powder coating your outboard motor would not be my first choice.

Some manufacturers looked at powder coating for their in-plant paint systems, but decided to go another way. Using a recommended liquid paint from the manufacturer may be the easiest. Let’s face it you are in a fresh water environment which is not as bad as saltwater. Powder coating may just be too much hassle for the slightly improved benefits.

By the way, after you disassemble the outboard, every part needs to be scrupulously cleaned and neutralized using distilled water. Aluminum substrates require a chromic conversion coating (alodine) if you want the best corrosion performance. This type of finish is used by the outdoor furniture market where 20 year warrantees are the norm.”

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