I used the very paint you're talking about on a big repair I did on one of my black Hobie racing rudders - paint worked really well and I didn't do much at all to prep the area I painted - just roughed up the new fiberglass with sand paper. Even though it's not for under water use I figure they meant the extended under the waterline keelboat sitting in the water all season kind of a thing. It's held up fine on the rudder, probably great for what you want to do.