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.