Spring Heater Box Cleaning - Damage Found

The problem with JB weld is that it does not have anything to back it up like the fiberglass matt, so it has no foundation. I don't know what the big deal is. JB weld is the same as fiberglass with one difference. You coat a backing matt with the two part mix. It's a no brainer to me to repair fiberglass with fiberglass. JB weld will probably crack again at some point in time. Fix it how you want to. It's your car.