I usually use thinned epoxy for fuel proofing the firewall and fuel tank area. Never really thought to use plain thinned epoxy to add strength Can't really see how it could add that much strength. You might be better off reinforcing weak areas with tri-stock balsa or lite ply or similar materials.

BTW Easist way I know to thin epoxy is with denatured Alcohol (Not rubbing since it has too much water in it). Others use a heat gun