Rusty keel
Hunter, like Beneteau and others, used cast iron instead of lead as ballast material on several of its models. Nothing wrong with it, just a little more maintenance. My 25.5 had similar small spots to what you have described. You can fix these the right way but it will take a little time, like 3-4 days.First, go to the Interlux web site where you will find a very good step by step guide to fixing the problem. Essentially you have to (1) haul the boat, (2)grind out the rusty spots with an angle grinder to shiny metal, (3)dab on a little rust killer (essentially an acid), (4) cover with a barrier coat like Interlux 2000 (then another layer of barrier coat and another layer of barrier coat), (5)then bottom paint, then another layer of bottom paint.The tricky part is when applying those several barrier coats, they must be applied within a window of time of one another which is determined largely by ambient temperature. It is longer between coats when the weather is colder. The same is true with the first layer of bottom/anti-fouling paint after the last layer of barrier coat, it must be done within a prescribed time period or there is a risk that the antifouling paint will not adhere to the barrier coat.If you are a serious racer, you might also want to apply a fairing compound to the keel where you have ground out the rust spots. Usually the area removed by the grinding just isn't that large or deep so I skipped the "filling and fairing" step after the acid and just prior to the first coat of bottm paint.If you have the time, you could do the whole keel and bottom with barrier coat. This will provide good blister protection as well as rust protection on the keel. I only addressed the those spots which were rusting. When I haul it again this spring I will be able to tell if this was the right decision. If I see more rusting, I will do the entire keel and the bottom with barrier coat.The bottom line is that it can be tackled rather easily with a sander, angle grinder, and good paint. Do allow yourself enough time at the yard to do it the right way. Good luck.