Moisture may be present in a wall cavity or above a ceiling, because of sweating pipes. If the moisture can escape, it'll affect whatever unprotected steel it comes in contact with. As a residential or commercial painting contractor, you're not likely to know as much about rust as your industrial counterparts, but you still can effectively coat it. Often it's just a matter of knowing some basic information. These coatings have rust inhibitors in them, and some are designed to bond tightly to rusted surfaces.
Primers have more inhibitors than topcoats, of course, because they're supposed to be applied first.
Primers are always an important component of the painting process, but they're of crucial importance when it comes to poor substrates. Primers formulated for rusted metal form bridge coats. They provide something for the finish coat to adhere to. Your main job is providing something for the primer to adhere to. Very light rust is something you can clean off easily with hand and power tools.