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Ask SAIL: Painting the Propeller

I read with great interest Don Casey’s advice (Ask SAIL, August 2013) on painting a bronze propeller with antifouling paint. He stated that it can be done, but not with paint containing copper. Why is this? What about bronze through-hull fittings?
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Juan Marquard of Mexico City, Mexico asks:

I read with great interest Don Casey’s advice (Ask SAIL, August 2013) on painting a bronze propeller with antifouling paint. He stated that it can be done, but not with paint containing copper. Why is this? What about bronze through-hull fittings?

I am particularly curious, because in July, after the bottom of the cast iron keel on my Beneteau Oceanis 50 had a close encounter with a small coral head in the Bahamas, I had it resealed with an epoxy barrier coat. I then repainted with Pettit Trinidad SR. I asked the boatyard at the time what they recommended for the prop, and they told me their practice was to paint it with the same copper antifouling paint as the hull.

Is there a problem leaving the prop painted as it is? My boat will be sitting in a marina for another six months without being used.

Don Casey replies:

The copper paint will not damage your bronze prop. Bronze is more noble than copper, so the galvanic interaction when the bronze prop is submerged dissolves the copper in the paint. The primary reason you shouldn’t use copper-based paint on a bronze prop is that the active ingredient in the paint is quickly rendered ineffective. It is nevertheless common for yards and owners to paint the prop and bottom at the same time with the same paint. If the prop is properly etched and primed before copper paint is applied, this can be somewhat effective. Typically, however, the paint simply erodes away in a short time, providing little, if any, benefit.

If there is a zinc anode on your prop shaft, the even less noble zinc might delay the erosion of the copper paint so that your prop might actually stay clean while your boat sits idle in the marina. Once you start using your boat, however, you should expect the prop to foul much sooner than the bottom, even though both are protected with the same coating.

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