The cracks I think your talking about is in the gel-coat of the fiberglass
I worked just with epoxy/resin fiberglass
while building canoes and know for a fact, if the area is clean of oils and wax this stuff will stick.
I used what is known as a fillet with is ground glass fibers mixed with the epoxy/resin mixture and filled and smoothed out many of little imperfections.
Spilled a dime size dollop on the cement basement floor and 25 years later it is still there in a high trafic area