Armor/Jetpack reinforcement

I've recently bought a used, painted JP, and it seems a little fragile to me. I'm wondering what the best (and easiest!) method to reinforce the jetpack would be? Would simply painting on fiberglass resin do anything worthwhile? How about a full fiberglass treatment inside?

Is there any risk when the fiberglass cures that it will shrink or expand and cause damage to the jetpack?

I build my jet packs out of fiberglass and resin. Using fiberglass and resin will not cause any expansion or shrinkage.
You can back fill with expanding ridged foam(the good kind). You may need to practice using the foam. You tube has a few videos on applying expanding foam.