repairing cracks in fiber glass is a pain. you have to grind out the crack. i do this with a 1/8 inch cut-off wheel on a die grinder and follow the crack.
then i fill the crack with resin, and skim coat with glaze (glaze is really thin bondo, spreads and sands very easy) usually no matter what you do, the cracks will come back, because it is very hard to see them all, and they only like to show up when every thing is painted..