If you are a JPO member Gordon Dickens just wrote an excellent article about repairing broken fiberglass in the latest Contrails newsletter.

It all depends upon the specific repairs. Cracks like yours appear to be, scuff the backside really well with a dremel with a ball ended carbide bit (1/4" to 1/2" ball) and roguh up with 80 Grit sandpaper, then glass it back with 6 oz cloth cut into strips, and either polyester or epoxy resin depending upon what the fuselage is made out of. (TGA is epoxy I think) the cracks on the outside black sand and fill with Evercoat polyeter glazing putty and sand smooth. If you have some larger gaps to fill you can use epoxy and microballoons, or sometimes even mix in a little milled fiberglass to fill gaps.