Is that just Body filler over the Break/crack?
This repair will be stronger or just as strong as the original panel was.
The Crack broken glass should be all ground down to a 45 degree angle all broken glass removed, This creates a big gap between peices Then on opposit side of 45 degree angle sand peices smooth and lay one layer of fiberglass cloth big enough to cover all the breaks/gap 2-3 inches soak it with resin and let it lay up then sand it down smooth and lay another peice over the top of the first at 45 degree angle from the first and soak with resin let it lay up, Then take Fiberglass MATT and pull it all apart so its not mat any more but a Bowl filled with loose fiberglass fibers then with fiberglass resin and fibers get a small brush and fill the gap on the opposit side of panel the side wich has the 45 degree angle on it and the gap, fill gap with Fiberglass fibers and resin fill Gap let it set up and sand down repeat as nessary till you have filled gap with Fiberglass fibers not just the resin let it lay up sand to finish paint to match. You want to use as much fibers as you can in the repair and only use the resin to wet Fibers so they are wet and translucent. This would be most correct way to repair broken fiberglass panels.
(actually the most correct way would be to cut cloth in small tiny peices in same shape of gap or hole and layer them over and over with peices of cloth getting bigger and bigger progresivly till gap is filled but you only see that sort of repair on Aircraft and some boats.)
Each time you let the fiber glass lay up you have to sand area you want to add more resin and glass too!
When the fiberglass resin kicks it has wax in it that forms a layer over the top of it to keep air from the resin for proper cure. So you cant just put fiberglass resin over the top of NON sanded areas it wont stick its very important that each layer be sanded thoughly and completly avoid strong chemicals such as Acetone on fiberglass panels you want to keep they can soften the resins.
Another thing about Fiberglass Resin when it kicks it gets HOT! Hot enough to start a fire so watch what you do with the resin cups make sure they have fully cooled before putting them in the trash.
Have fun
Oh good trick when playing with Fiberglass Rub arms down with Baby powder first!
To remove glass from skin get a old pair of Wifes Nylons rub skin with it