We have had a problem with rusting of our keel from when we bought our B34 in 2002. She is not copper coated. We employed a water blasting company in 2004 to blast the keel down to bear metal. The hull was protected local to the top of the keel and it took two men to hold the water lance. You could not have cleaned the rust off more thoroughly. This was a major industrial process. We found that the casting was very poor and there were lots of slag deposits left from the casting process. Some of these were up to 25 mm deep and similar in diameter. You are probably experiencing rusting from within the surface of the casting as it is impossible to clean back to clean solid metal. We coated ours with six layers of primer similar to Primacon but thicker, but we still get the rust coming through from below the primer. I think its an ongoing clean and patch each year as any system you apply will simply be locking in the rust behind that is within the casting slag.