I would use straight West epoxy
on the keel after grinding it to bright white metal. Put five coats on, then do whatever fairing you want. Straight epoxy
has worked for me on steel keels, I do not think the Interprotect is strong or hard enough to coat steel for the long run. Also Watertite is a bear to sand and fair, and I dont believe is intended to fill large voids, use it for underwater divots, not above the waterline. I'd use an epoxy
/silica mix for the keel joint. be neat as it is also a bear to sand.