cast iron keel coating
In terms of preparing the keel, I prefer to have the keel sand blasted white, and then immeadiately apply an epoxy coating. In industrial applications, ferrous metals are generally coated with zinc-rich epoxies.(epoxies with a high zinc content added.) These form a chemical as well as a physical bond and make a great primer. If you choose the right zinc-rich epoxy you should be able coat the primer epoxy coats with a conventional epoxy such as WEST System or MAS epoxy (I prefer MAS for a variety of reasons but WEST is a good product.)
If you choose to skip the priming step then you can use a good quality epoxy such as WEST or MAS right on the iron. WEST actually has a good step by step decription of the process. I personnally like to use a layer of cloth any time that I do anything like this. I think that the resin had poor tensile characteristics and so is more likely to get small hairlines that let moisture in over time.
In terms of fairing the keel, by far the worst part of this whole business, I typically use a trowel that rectangular notches about an 1/8" deep and an 1/8" on center. I apply a first fairing coat of thickened epoxy with this trowel carefully getting reasonably vertical stripes. It is easier to work these down to fair first with a Red Devil body working plane and then with a long board. It is much easier to sand these ribs fair than a large flat surface. Once I gave these ribs close to fair,it is easy to fill in between the ribs with a tickened epoxy filler. I try to overfill the ribs and let it cure for a while after setting. I also dye the filler so that I can see when I have sanded down to the ribs. When all of that is done I then seal it all with a couple coats of epoxy as a barrier coat.
I check fairness with a batten and if I really care I make up a template by making a rough shape in plywood that I coat one edge with body putty. I tape saran wrap over the area that I want to match BEFORE I STRIP THE KEEL(I have on occation taken an impression on a sistership with permission) and press the body putty against the Saranwarp to get an impression that can serve as a template.
Done right this will our last any of us.
Good luck, I don''t envy you this project.