Quick answer - depending on country/time period/aircraft etc, then yes, most aircraft would have an etch primer applied. Many modellers show bare metal, deduced form photos, when, in fact, this could be either the primer, or, more usual, the base layer of the top coat paint, worn through and looking lighter in tone. However, it's important to know the subject, and those areas where wear and tear would result in bare metal showing, before applying such to a model. Areas of heavy wear could, and did, show bare metal, for example, around the windscreen frame, where constant abrasion by hands could remove the paint, and prime, and show vague signs of bare metal. Obviously, where 'metal to metal' contact abrasion took place, for instance around canopy tracks, then yes, bare metal could be expected.
Bottom line - know the subject, deduce areas of heavy wear, and study every image of the actual subject aircraft being modelled.