I primed them, then a wet sanding. After that I shot the first coat of color, then wet sanded. I repeated for 5 coats. it is fairly easy just go in light strokes. don't hold the spray gun in one spot or go to slow or it will get thick & look like crap...
One bike I did, i made it snake skin. I shot it 2 coats of gloss black. then I put on a fishnet stocking on the tins. Used the airbrush to spray on 3 shades of green. 1st was the lightest shade the whole surface area. the darkest was right along the stocking threads.
I started using an airbush in my early teens on model cars and other art...
like everything it takes practice...