T-M, in respect of `hoar frost` on the underside of the wings,it would depend on whether the aircraft had `integral` tanks,or self-sealing tanks,the latter providing a measure of insulation from the wing undersurface.Also ,on the time of year,as in winter ,a descent from altitude to a sub-zero airfield,would probably not allow the `frost` to sublimate away,especially after a 5-8 hour sortie; that would also depend on how much residual fuel was left in the tanks.