I don't know if I was doing something wrong, but I was just going to seal some leaky rivets in my old aluminum duck boat and the putty wouldn't stick to the boat. I got some steel wool and roughed up the area, which helped a little, but for this application I found it was easier to use j.b. weld or some kind of epoxy. Maybe it would work better on fiberglass. Don't get me wrong...it dried very hard, and it hasn't leaked since. It was just not very sticky, that's all.