I’ve hung around a lot of real-life natural metal airplanes, some of them are show planes that have been highly polished. Kind of depends what the aircraft is and the method of construction but as you’re doing us English Electric Lightning fighter, I doubt you’re having to deal with lap joints. With jets, even well polished maintained one’s, I’m not sure that there are a lot of panel lines to be seen, unless of course you’re dealing with Soviet things that were basically assembled with boilerplate. Panel lines on most Jets, especially supersonic jets, are almost non-existent from a normal viewing angle. However, access panels do leave some noticeable gaps, sometimes visible even from quite a ways off. However, even those stay fairly clean. You get back towards the engine area or where they work on hydraulics, you’ll start getting residue, very very fine layers of grease and oil, which then collects soot from the engines. I think paynes-grey is a great idea, although you get farther back to some of the aft areas and also the underside of the airplane, you can get away with actually using black, or a very dark brown. However, dilute it, or else from normal viewing distance, the intense dark of the washer against the shiny natural metal will play a trick with the eye and make it look more like a pencil drawing in a 3D model.