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.