When we had our new-to-us boat trucked home from SF, there was an indentation the size of the small baseball in the bottom of the leading edge. Quite a 'divot' in the lead. The yard ground it back a little where the lead was pushed out around the 'crater' and filled it with something, and it was likely epoxy-based filler. The area is still indistinguishable from the rest of the keel today. You want to get the surface clean and roughed up to give tooth to the filler.
Admittedly it's easier to get a filling to stay if the sides of the cavity are steeper, as your dentist proves when he/she drills to form a proper cavity for the filling material.
IMHO, 36 grit to rough up the lead, and immediately apply the filler before any oxidation can occur.