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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.
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.
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.
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.
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.
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