When I did the iron keel on my Columbia 43 I had it sandblasted and immediately coated with epoxy (not WEST - too expensive & no better than all the rest).
Because the iron was pitted, casting flaws exposed etc. I wanted to fair it up before putting a finish on it. I coated it with epoxy & talc, thickened to peanut butter consistency, using a 1/4" notched trowel. This left a surface 1/4" deep but full of grooves down to the keel surface. I then went over it again with a smooth trowel to fill in all the grooves. The result was a uniform 1/4" deep coat of filler. If you have the skill of a plasterer you can do this in one pass but I don't so this gave me a uniform depth.
I used talc as the filler on the recommendations of the yard owner - an old salt German who started life as a journeyman plasterer. It's dirt cheap - a cement bag of it was about $20. It also provides a beautiful surface for sanding - not too hard and leaves a very fine surface. I now use it exclusively for any filling that requires a "finished" surface.
Sanded everything with a 16" sanding board from an auto body supply shop - it uses pre-cut strips of 17 1/2" sandpaper. The board has a handle and knob like a hand wood plane and the strips of sandpaper clip on at each end.
Sand the filler diagonally, as you would any compound curved surface - car body, boat hull etc. Depending how corroded your keel is, you may have to do this process more than once. Be careful not to sand through to bare metal - if you do, IMMEDIATELY coat it with epoxy or filler to prevent oxidation from forming.
Once you are satisfied with the surface, wipe it down with solvent - acetone, MEK - whatever you prefer. Now you are ready for the final finish coats. I chose three coats of epoxy resin followed by two coats of InterProtect. This was mainly due to some rather widespread disagreement as to what was best at that time. I think now that I would use just Interprotect.
Now you're ready for bottom paint and if you were diligent during your filling and sanding, you should have the next best thing to a templated keel.
P.S. Grinding an iron keel isn't good enough - must sandblast. Every single iron keel I have seen hand prepped via grinding and wire brushing etc. has had a limited life before needing it again. Sandblasting means it will last WAY longer.
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I, myself, personally intend to continue being outspoken, opinionated, intolerant of all fanatics, fools and ignoramuses, deeply suspicious of all those who have "found the answer" and on my bad days, downright rude.
Last edited by SloopJonB; 12-12-2011 at 08:34 PM.