Here is a look at the keel with filler. It's been sanded to shape, and the first layer of fiberglass has been applied.

I made a mistake here... Thinking that this was basically new construction, I used polyester resin mixed with microballoons as a filler on one side of the keel. After it had cured, it shrank a bit and pulled away from the keel in a few places. At this point, all I could do is fill the cracks with epoxy and move on... I used epoxy for the rest of the keel. Its greater adhesive strength held the compound tightly to the keel, and the subsequent layers of fiberglass got a good bond as well. The moral of the story... use epoxy and you'll rarely go wrong, even though it costs more.