What I used
What I used where I replaced the keel on my 27 was Bondo's all purpose house fller. I contacted Bondo's tech support people on using thier product for this and they assured me it was waterproof and strong. The Bondo automotive filler will absorb water. Thier filler is nothing more than polyester resin with microbubbles and talc. I also found this filler was easier to sand and much, much less expensive than any marine filler. Also the boat has been back in the watyer for a year now and no flaking or cracking. I did overcoat the filler heavily with a barrier coat just for insurance. Check out my website and see how I did it.