You need to refer to standard fiberglass repair techniques. Typically, you would try to grind a "v" shape in the cracks-though my personal experience is that the cracks are ususally shallow-and grind off easily. The one which leech fuel are through the tank-grind a v-fill with resin and cloth-grind smooth repeat as needed. I never liked any of the fiberglass "trick fillers" myself, prefering to use resin and matt-then plastic filler over the corrected glass work to fill small imperfections, pinholes etc.
