I've had two boats with iron keels. Both were primed and painted originally and held up well. On one I scraped scaled and sanded it by hand, primed with zinc paint, and copper bottom painted. That was over 20 years ago, it still looks good. I have no doubt that galvanizing is best, but many thousands of steel ships travel the world with no more than paint and primer on the hull. Many many classic boats have iron keels that are just painted too.