Duh, the original poster said he's fairing his keel. So, in that case, there will be glop all over the original gelcoat. You've got your choice of paint for a drysailed boat. Plain old alkyd enamel paint (Rustoleum is such a paint) will do fine. Or you could go with a fancier one-part polyurethane (epoxy) paint, like Pettit EasyPoxy. (Product name sounds like it makes it easy for you to catch chicken pox, doesn't it?
) Then, you can move up to a 2 part polyurethane, which may or may not be worth the expense or effort.
If you're restoring and fairing a cast iron keel, then you should consider the following schedule: clean as much rust off as possible. If there's rust scale, beat with hammer, needle gun, or have it sandblasted. Coat with 2 coats of Ospho phosphoric acid, to create a layer of non-rusting iron phosphate. Then coat with perhaps 2 coats of coal tar epoxy - a formula specifically for marine application. This is what the USCG uses on buoys. Then, you can fill and fair, and paint to your satisfaction. During the fairing process, if you sand though, fix it with the Ospho and CTE. This is how to make the most salt-water resistant cast iron below water appendage. And you need to go in that order, otherwise, rust will form under your fairing material, etc.
