How are light weight fillers made for fixing dents on airplane wings? Say an airplane wing was damaged by hail, it's really costly to replace the panel due to inaccessible rivets and what not, and the back of the panel is inaccessible to mechanically remove the dent.  How are light weight fillers made to fill the gap? Not like the heavy stuff like Bondo that is (was?) used to fill dents on cars, piled on and then sanded smooth.  Airplanes need something light weight.
Lightweight body fillers such as Poly-Fiber's SuperFil are two-part epoxy fillers. The reason they are lightweight is that a significant portion of the filler in the epoxy is composed of microballoons, also known as glass microspheres.