I've done several re-fairing jobs on cast steel
keels, using west system and sawdust, with micro balloons. The stuff sticks like crazy, and when it was finish sanded to contour I used Interlux
2000. 4 coats of it, and I never had a problem with it again. I've also removed a few through hull
fittings, beveled the hole and filled it with cloth and west system. Again, when that was done, it got painted over with Interlux 2000.
I now have a steel
boat and was about to use the Interlux on it, but one of the club members who also has a steel boat laughed at me and recommended Amerlock 400. Stuff is tough as nails, and will cure at temps as 0C
Here's the PDF for it. Since I'm working on my boat in the dead of winter, I'll be using the Amerlock.