While sanding the bottom, I had the chance to investigate a crack in the fiberglass where the rudder shoe would be located under the keel. I was concerned that the crack may be a sign of corrosion in the bronze rudder shoe, which is typically protected under layers of fiberglass.
Sanding away at the epoxy barrier coating near the rudder, I discovered two things: first, the rudder shoe is fine, with no corrosion. There are a few small voids in the fiberglass filler around it, which I will fill-in before glassing over the entire rudder shoe once again.
Second, as I was trying to sand out the last remaining patches of stubborn antifouling, I discovered that the previous owner (or more likely someone he hired) had put epoxy barrier coat over the red ablative paint on the rudder. Not sure how it managed to stay on for so long. I will have to sand the epoxy barrier coat off entirely.