Grant
31.05.2021 5:18:05
CTO - Jackson, Michigan, USA here
See full version: Best way to paint a cast aluminum boat stern drive
CTO - Jackson, Michigan, USA here
Ludovic Dumont
- Paris, France
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Several paint manufacturer salespersons told me that zinc-rich primer is not suitable for aluminium, but I can't find any good explanation why.
Zinc is a bit less noble than aluminum, so galvanic corrosion should protect aluminum. And as they are not very far in the galvanic scale, what I understood is that zinc should even protect aluminum longer than steel (for example), because the galvanic corrosion would be less intense. here
Ted Mooney , P.E. RET
Aloha -- an idea worth spreading
finishing.com - Pine Beach, New Jersey
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Q. I would like to know of a preferred method of painting or coating my Volvo outdrive lower unit, which is aluminum casting.
Follow the primer with a several spray coats of acrylic lacquer and the outdrive should be well protected for a few seasons. The process is the same for repainting outboards. Readily available spray cans of both the primer and the top coat in factory-match colors make paint touch-up relatively easy for the do-it-yourself owner.
Where the old paint is bubbled or peeled, you must remove all loose paint along with the underlying corrosion. I generally wet-sand exposed aluminum with 80-grit wet-or-dry paper. If the aluminum is pitted, you will need to use a wire brush to remove as much oxide as possible from the pits. After you clean the bare aluminum, fill significant pitting with an epoxy filler such as Marine Tex. more
When the aluminum is clean, oxide-free, and completely dry, give it a couple of coats of zinc phosphate primer. This primer etches the bare aluminum for better paint adhesion. Sanding the primer likely will not be required, but be guided on this by the instructions on the can. [links]
As with all paint projects, the key is preparation. For recoating paint in good condition, you only need to wet-sand the old paint with 120-grit wet-or-dry paper to provide tooth for the new topcoat. If there is a chance that the outdrive has ever been waxed, wipe the entire surface with a dewaxing solvent before you do any sanding.
If your old paint is in bad shape, perhaps due to underlying corrosion, you need to repaint rather than touch-up. This is a bigger job, and for it you will need to do a better job of preparing the metal than spraying it with phosphate primer. Successfully painting bare aluminum, especially when it will be submerged in seawater, is an exacting process. Start by scrubbing away all corrosion with a Scotchbrite pad. Sand and fair any paint not removed with 120-grit aluminum oxide sandpaper. For your new paint to have a chance of staying on the submerged drive, the bare aluminum must be chemically etched The usual products for this are either Alumiprep 33 or Metalprep 79. After that you need a conversion coat — Alodine — followed by an anti-corrosion epoxy primer. Only now can you top coat with a paint suitable for underwater service, including the color-matched spray lacquers available from dealers and marine supply stores. Be aware that skipping a single step will result in early coating failure.
Ensure any waste product including cans, rags, brushes, roller sleeves masking tape and paper, gloves and overalls are disposed according to local regulations. Do not allow any waste material such as unused paint to enter any waterways. more
Drive shafts, trim tabs, struts and other fittings may be manufactured out of stainless steel, bronze, or steel.
Aluminium must be coated with an antifouling specially formulated for such surfaces.
The key to protecting underwater metals from corrosion is correct preparation of the substrate and choosing the best priming solution for your project. But before you start the preparation, you need to establish the exact type of metal you are working with. here
Propellers are manufactured from bronze, stainless steel or aluminium. more
Due to the severe turbulence and adhesion problems relating to bronze material we do not recommend the use of any specific primer at this time
The best anodes are made to military specifications and some will have "mil-spec" stamped on them. If they don't, make sure you buy only the best-quality anodes from a reputable marine chandlery like West Marine. Cheap anodes are more likely to contain impurities that will render them far less effective. Even the best anodes must have good contact with the metal they're protecting, or they will be ineffective. That means they can't be installed over painted or dirty surfaces. A quick once-over with some fine sandpaper on the metal helps maintain contact. Once installed, anodes should never be painted because this will deactivate them. Finally, anodes have to be replaced when they're about half gone, because with less surface area, their effectiveness is reduced.
The damage to the outdrive in the picture is severe, likely starting with a few bubbles in the paint, the first telltale sign that something was wrong. Corrosion, especially on aluminum, changes the surface of the metal so that coatings no longer adhere. As with most marine-related corrosion, regular inspections can head off major problems. Examine your outdrive frequently, and if you find bubbling, peeling paint, or pitting, don't ignore it. Take steps to correct the problem before your outdrive crumbles. (Corrosion isn't covered by insurance.) Now let's look deeper: more
On the positive cycle, corrosion occurs, but on the negative cycle, some re-plating occurs so corrosion can take significantly longer. However, it's believed that in swiftly moving water, as in a marina with current, the re-plating may not always occur, and AC corrosion will be accelerated. more
The outdrive is then connected to the neighbor's grounded underwater metals (even one several boats away), which may be stainless steel or bronze. Because the two different metals are in the water and connected through the ground wire, it's the same as if they were in direct contact. If a boat's underwater metal isn't protected by an anode, it will use a neighboring boat's anode for protection. When that's gone, it will use other less-noble metals, likely an outdrive, to protect itself. The solution is to install a galvanic isolator, which will prevent low-level DC current from flowing through the green wire — effectively breaking the connection to neighboring boats — but still allowing shore power current to flow to ground in an emergency. [links]