You are probably missing that antifouling paint
right on the bottom of the keel
where it has hit the bottom as in groundings. Even if you 'prime it' with something, it will eventually happen again that the paint
gets rubbed off anyhow at the keel's bottom.
There are barrier coats, epoxy
coats, black tar coats you could use as 'primers' if on the fiberglass hull
itself, or just paint with several coats of the regular antifouling you have on the rest of the underbody , but......
The trick is to....
1- Sand off the surface to solid whatever you can get; be it shiny lead, prior paint or primer or whatever mixture of spots and colors you end up with while trying to get the exposed lead parts
somewhat shiny. Bottom paint
will stick well to the rest of the spots except to the exposed lead if it is oxidized or dull. Make sure you use a mask and gloves because lead is very poisonous. Well; so are all marine
2- When you have the brush or roller in hand, have a piece of rough sandpaper in the other. Apply some paint only on the semi-shiny lead spots ( Lead dulls very quickly exposed to the air. ) now; sand the paint or primer or whatever you are applying, while wet, right into the exposed lead parts
so you are actually getting paint into the very un-oxidized lead itself, ( like applying and rubbing the paint or primer with the rough sandpaper!) cutting off the oxygen to the metal before it oxidizes again and even now while wet, paint all the other spots evenly as if all one larger area. Let dry some and apply as many as 3 or 4 heavy coats of antifouling if this is the bottom of the keel, allowing some drying time between coats. This way it should last and not peel. Not much fun but it is satisfying when done.