How do you fair a keel?
I agree with Paul that you don''t need to go to the extremes that racers usually pursue but you should do some fairing. When you sandblast the keel the cast iron surface will have a distinct resemblance to the craters of the moon.
(There used to be a good description of the fairing process on the WEST System Epoxy Website but you may have to order their Monograph via snail mail.)
The first step in fairing is to get a well adherred first coat of unthickened epoxy on the keel as soon as the sandblaster stops. As soon as the sandblasster stops blasting, wipe down the keel to remove the dust and roll on a coat or two of epoxy. These days I am using MAS but WEST System is very good as well. (Now here is why I like MAS)You then need to wach and scrub the whole area with an 3m pad to remove the ''amine blush''. MAS makes a product that does not have an amine blush.
My favorite way to fair a keel (If there is such a thing as a favorite way to do an onerous job like keel fairing), is to use thickened epoxy and notched squeegee. There are squeegees that have 1/8" wide notches every 1/4" or so, that is what you want or need to make if you don''t find one. Typically I trowel on the thickened epoxy and then run the notched squeegee vertically so that I end up with a series of near vertical rectangular ribs. There is a point before the epoxy had completely cured (just at the end of the rubbery stage) when it is easier to cut the epoxy. Using a RedDevil, Surform bodywork plane, I plane down much of the excess material. I use a batten held fore and aft and on a diagonal to look for humps, bumps and hollows. Don''t take too much off with the plane. Once the epoxy has cured I grind the ribs down to a final faired shape.
You then need to wash and scrub the whole area again with an 3m pad (including the bottom of the grooves) to remove the ''amine blush''. When you are sure you have the shape that you want and the amine blush is off the entire epoxy surface, fill the grooves with thickened epoxy as well. Some people use a different colored filler for this because they like to slightly overfill the whole area and when the ribs show up they know to stop sanding.
Once the new filler has been ground down to the top of the ribs, check the fairness with a batten again. If there are areas that need minor attention, you can spot fill and sand just those areas. When everything is too your liking you should roll on a barrier coat and you are ready for bottom paint.