keel/hull joint;
I had the same condition on my 1990 30+, only mine was leaking. What I did was brace the keel in position, loosened all the keel bolts, lifted the hull aprox. 3/4 in and cleaned the hull & keel faces. Then I applied aproxamately 6 tubes of 3M 5200 to the faces, lowered the hull & retightened the bolts.
If your joint isn't leaking yet, I would first a) tighten all the keel bolts (150-225 ft.lbs.of torque), b) clean the crack out as best I could with scrapers & sand paper, c) power wash the crack & dry throughly, & d) fill w/5200. I did this on my E34 that I presently own and it is working fine.
Glassing over the joint won't help as there tends to be a little movement at this point (OK) and anything that isn't flexible will just crack & fall off. As long as the surfaces are clean & dry, 5200 will bond to them and give you the flexible sealing surface you need. Basically it functions like a gasket.
The movement comes when you heel over and 4000 lbs. of lead is attached at a point 6" wide and tilted at 20 degree angle. The engineers on this web site will probably be able to figure the exact numbers, but the point loads are incredible and to try to make it rigid enough to not flex is almost impossible. Better to let it move and give it something flexible.
All keel bolts have a tendency to loosen over time. The best way to check tightness is with the boat hauled and the hull weight resting on the keel. This way you're not trying to tighten the bolt AND lift the 4000 lbs. of lead at the same time. Give yourself a break, man. It's tough enough as it is. Basically, as tight as you can make them (unless you can bench press 400 lbs.) is tight enough. Twisting off a 3/4 dia. bolt by hand is almost impossible for normal humans unless you're using a 6 ft. piece of pipe on the end of the wrench.
Hope this helps you out. The first fix I laid out is a worst case senario only. Mine was leaking faster than 800 gal/hr so a drastic fix was needed. I recommend the second way for most conditions.
Good luck & sail fast
Bud E34 "Escapade"