I don't think you can use anything hard, like epoxy. Whatever you use probably needs to flex. Be waterproof, and something that will take paint. I think this is referred to as the "Keel Smile". I cleaned up the area as well as I could with tools and a Dremel, opened up the crack just a bit, I didn't want to open it up much, just enough to get to good material. I probably went in 1/4, maybe 3/8 of an inch or so. I could see where I was getting to the original Thikol stuff the boat builder used at the joint. Once I had it all cleaned up, I wiped it down with acetone, shot with compressed air, then applied several coats of 5200. Each coat would take a few days to dry enough so I could sand it. The first coat or two I just layered in, no sanding here. The stuff will sag, so you can't apply that much, I did mist it a bit to quicken the cure and reduce the sag. I taped off the surround areas. I had plenty of time so I used the regular 5200, not the Fast Cure. I suspect Fast Cure would be fine, if you needed to move the project along faster. I think I used 80g each time I sanded, then the last coat or two maybe 150 or 220 ? For the last coat or two I smoothed out the just-applied 5200 with a plastic scraper to get it to fair / blend in nicely. Once I was happy with the results, I applied several coats of 2000e Barrier Coats over that, then a few coats of bottom paint over that. Hopefully it worked, we'll see this summer.

I should note I was fairly certain this was cosmetic. I have had no signs of leaks in the bilge. No indication whatsoever of any keel bolt issues or anything structural. The crack was "hairline", barely visible, at the keel/stub joint at the very front, leading edge of the keel, going aft about 8-10 inches, on both sides.

Cheers, John