^to possibly answer your question you dont necessarily HAVE to derivet to pain the whole thing - however, to have the best results you're gonna have to to be able to paint the spots covered by the riveted planes of metal.
sure you can just paint over what you see, and it will be fine. it may be hard to maneuver yoru way around the case without having taken apart the whole thing first though (which, of course, requires you to derivet!) good luck