Join Date: Nov 2009
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Haven't dealt with cast iron, but had a steel centerboard for 16 years. When I bought the boat the original owner had coated it with Rustoleum and--not knowing any better--I pt bottom paint on and launched it for the season.
What a mess when I hauled for the winter! The Rustoleum failed, taking the bottom paint with it. The centerboard was exposed and further pitted all over.
The only thing that worked was having the board sandblasted and coating it immediately. The best treatment, when it became available, was Interlux 2000. I put it directly on the clean metal and built it up with several coats. It only failed where it got dinged when I had been "sailing by braille" in shallow water.
Subsequently, I have used a SS wire brush wheel on aluminum that was below the waterline. I just couldn't get to clean metal in the pits. I applied 2000 after using a zinc chromate primer and it held where there was no pitting.
The lesson learned: forget scraping and brushing. The only was to get to clean metal was sand blasting. I doubt soda blasting is aggressive enough to get all of the pits. Once you get to clean metal, IMMEDIATELY coat it. Zinc Chromate primer (thin coat) on aluminum, followed by 2000. Use 2000 directly on bare (shiny) iron or steel.