Usually, I'll hinge the surfaces after the final primer coat and then spray the base, trim, and clear coats on the assembled model.
The trick I learned is to spray across the hinge line, never directly down into the hinge line. As you pass the gun along the span of the wing (or stab), just cock your wrist slightly so the gun is angled away from you. When you hit the hinge line, you will spray at a slight angle and not get too much paint build up in the gap. I use Klass Kote epoxy and a few light passes are all it takes to get good coverage.
It just takes practice. Hope this helps.
Ron