I have repaired the rudder on my 1991 Hunter Passage 42 in that fashion. I talked to the people at Foss Foam, who manufactured my rudder, to ensure that it was made with closed cell foam. I am not sure if your rudder is similarly constructed, so my experience may not be transferable. The Foss Foam folks told me that with closed cell foam there is no danger of water intrusion getting to the rudder stock and corroding it inside the rudder. In my case the fiberglass/gel coat came completely off, and I repaired it by reglassing and painting it. I did that about 13 years ago and the repair worked fine. Good luck.