How do you fair a keel?
This is the sort of thing the racers go nuts over- what is the optimum radius for any "corners"; balancing wetted surface with added weight. You will be probably be fine using your (gloved) finger to fill in and round off the joint -- like the seam between a bathtub rim and tiled walls. If you do a nice job, follow-up sanding will be mimimal. Theoretically, you don''t want any hollows or grooves that would cause water to eddy around in them and slow you down. You''re asking all the right questions. Now it''s time to get to work so you can go sailing!!!