Gene - I used epoxy, but mixed it with milled fibreglass and had no problems so far (over 130 flights with 30cc petrol engine).
If I was doing it again, I will be using Hysol epoxy instead of the "normal" 30 minutes epoxy, just due to the fact that it bonds even better with the fibreglass.

Microbaloons form very stiff contact surface which will not be suitable in the high vibration environment as is next to the engine.
Saying that, the bond should have hold at time of construction unless you applied force in direction that the bond didn't hold.

Cheers,
Eran