ASA and PSIA Instructor
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Based on my experience with a First 305, if you only paint the keel, whether grinded (sp?)or etched or not, once it goes back in the water, the keel will resume the same appearance that it has before you painted it, i.e. rusted metal quickly re-establishes itself. Other than looking good for launching, the paint and prep work is of little use...
The bad news is if you want bottom paint permanently on the keel, the steel needs to be covered with multiple layers of epoxy (I believe West Systems probably has specific instructions for this application) and can then be maintained like the rest of the bottom.
The good news is this is also a good time to fair and shape...If a Previous Owner hasn''t finished the keel properly, then you you get the chance!