Barry-
Go to the MAS website; they have a formula for mixing fairing compound. If you only use glass strand it might be too tough to sand smooth. When you mix the filler you are going to whip in a bunch of air anyway so I don't see a huge issue with the microbaloons or silica. The thoroughly mixed
epoxy will prevent water penetration into the filler materials.
Sand the fairing compound within 24-48 hours after it sets. If you don't it might be tough to get it to sand flat to the hull. Don't sand off all the
paint that is still on the keel; it is more difficult to get the new
paint to stick to the lead than the old
paint. Scrub the areas of the lead where it is exposed with scotchbrite and immediately apply some barrier coat
paint (or
epoxy).
On the rest of the hull; be sure that all of the anti-fouling is removed. If there is some barrier coat
paint I don't think it will matter much since you will be applying a new barrier coat anyway. If you are going for a racing bottom finish you will probably want to sand the barrier coat smooth before topcoating with a smooth finish anti-fouling
paint.