I didn't really know what to do either, so, at Scamp Camp, I talked with John and Simeon and Howard to get some ideas before tackling the (kit CNC-cut) foils. Their instructions were to sand the CNC ridges all down to the bottom of the "valleys". That's the desired shape, remove all material to that point. I used a long board with a very coarse grit to knock most of it off, and finished at 80 grit before glassing. One layer of 6 ounce E-cloth, plus an extra layer on the bottom edge only. Like Mike says, a bit of fairing filler in the 2nd coat of epoxy, sanded with 80 grit, one more epoxy coat, lightly sanded. Then I used three coats of the same paint used on the hull (Petit Easypoxy), sanding with consecutively finer paper between coats, maybe ending with about 200.
The centerboard looked fricking beautiful when I dropped it into it's case three years ago, and I haven't seen it since! (The rudder looks just as beautiful, like a race-boat foil, and I get to see it all the time!)
Like Mike, I sail in fairly-cold salt water in the pacific northwest, but haven't left her in the water very long, 10 days at most. One time I had tons of baby barnacles all over the hull after 2 days! (no idea what's on that CB, but at least it still goes up and down