Just say no to thinning epoxy.
While I agree with the above sentiment for the applications mentioned, thinning epoxy is very appropriate for areas that aren't using the epoxy for strength, but where it is being used for fuel proofing and waterproofing, such as firewalls, fuel tank areas and the fuselages, hulls, floats, etc. of float planes. Thinned epoxy is very good for these areas and adds less weight than the heated epoxy method.