Outdrives are difficult to keep free of growth. The main reason for this is that traditional copper-based bottom paints cannot be used on an aluminum outdrive. Doing so will result in serious corrosion from the aluminum reacting to the copper in the paint.
When painting an aluminum outdrive you must use a copper-free bottom paint. There are a couple of different brands out there. Interlux Trilux 33 (The most popular) , and Sea Hawk Smart Solution (The best). Trilux is the most popular and easy to find at your local West Marine. However, the results are not as impressive as Sea Hawk Smart Solution (available in pint size black as Outdrive Paint) that uses the newest metal free technology. You will get the best possible results with Smart Solution, and is becoming more and more popluar as the best solution for painting an outdrive. It is also the best priced of the two, but is yet not available in an aerosol spray. You can get a quart and brush it on, or you can spray it with a conventional spray gun.
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How to Apply Bottom Paint to an Aluminum Outdrive