if you were airbrushing you were spraying with too high of a pressure, not enough paint coming out, or too far from the model (or a combination of any of those). Try adjusting one of those at a time and see how it comes out. What pressure were you using? typically unless I am spraying future I don't go above 20-25psi, you want sufficient paint coming out of the airbrush to sort of form an oval of paint when it hits the surface, not a dry grainy like texture, and you want to be within 3"-4" or so from the model when you do an all over coat.
If you were using a can of spray paint, move the can a little closer, within 6" to 8" (it usually recommends a distance on the side of the can) and move in a steady motion (don't go too slow or the paint will build up too thick)
You can sand the orange peal off with progressively finer grades of sandpaper and shoot it again, or use something like easyoff oven cleaner or Castrol Super Clean to soak the paint off. (if you go this route remember, the paint on the inside of the model will come off too and will be very hard to put back on!)