You're right about the gloss black. Airframe aluminum is one of the "high shine" finishes and needs the black undercoat.
What kind of black? Acrylic, Enamel or Lacquer? Did you use anything on the black undercoat: rubbing compound, polish etc.? I use gloss black lacquer exclusively, and I have polished the undercoat to get a good surface--after it cures--and as long as you clean off any polish or compound before the final coat it should work. I usually wash with a very diluted detergent solution and rinse thoroughly.
It sounds like the paint is drying before it hits the surface. Spray fairly close to the work and use a low pressure. I usually spray Alcad II at 10~12psi, with the airbrush adjusted for a low paint flow. Spray several light coats.
Good luck, and I hope this helps.
Regards,
Bruce
"You can't have everything--where would you put it?"