Badger used to make (may still make) a proper double-action airbrush, capable of taking a small jar-type resevoir. IIRC, it was called 'Tornado', and was popular (perhaps intended for) those who did T-shirt decoration. Good enough coverage for 5" gauge loco components, I would have thought, and capable of reasonable fine work too. It was sold at what seemed an almost bargain price.
It's not considered a good idea to put etch primer through a good-quality airbrush, but I've used a cheapo single-action airbrush for the stuff without destroying the thing.