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Join Date: Sep 2005
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Cast iron keels are prone to this sort of thing. Best way to deal with this condition is to epoxy-seal the whole thing. This involves sandblasting or otherwise removing all the paint (down to bare metal) and quickly sealing with a sealing epoxy. (S3, IIRC)
Properly done, this will prevent rust from appearing for several seasons at least, unless the coating is broken from contact with debris or the bottom.
If you choose to remove the paint yourself, be sure to use proper respiratory & body protection and collect and dispose of the debris as required by your local laws/yard rules.