Yep, what dave said!
I have done a lot of glass and smc work on vettes, bedcovers and toppers. The best repair is if you can get to the backside and grind the back broken strands, sand the area to accept the repair matierial with 80 grit sand paper, blow off dust and apply backing patch. You can use fiberglass mat or cloth with resin or what we use most of the time- fiber reinforced tape(like drywall mesh) and 3M's fiberglass and smc repair material, that comes in a 2 part mix tube with nozzles and requires a special gun to use it.
The next part will work after reinforcing the backside or if back is not accessible.
- grind the cracked area out in a V gradually working out from initial crack and grinding out as much of the broken area as you can( if hollow on backside and not accessible leave a thin layer of the broken matt to hold the new material from sinking in to far). And when I say V, I mean feather it out from the center of crack an inch ir so. If you don't feather it out it will not bond to a large enough area to hold and flex with original panels and will crack out again and again.
- After grinding, sand around the area getting repaired with 80 grit at least as far as material is going to be applied.
-now you can fill in with long strand filler, fiberglass matt or cloth w/ resin cutting small pieces and building up with bigger and bigger pieces as you work out.(matt and resin is hard to do on anything but horizontal areas) or like I usually use any more, a little piece of fiber reinforced tape and 3m smc and fiberglass repair material.
- when any of the above has hardened you can grind or sand down. And any further build up can be done with the same or short strand filler or regular body filler, and any pin holes can be filled with glaze.
-final sanding should be done with 150-220 grit and then can be primed. Sanded with 320 grit and then sealed and refinished.
--- I recomend a resperator, latex gloves and long sleaves any time you are grinding and sanding- and of course safety glasses or face shield.
Sounds complicated but someone with a little experiance could knock it out in an afternoon- less primer drying and refinishing.
Hope it helps any ? Just ask!!