I'm taking on a huge bike rebuild here soon, to include a full paint job. It's going to be a bit more work but I'm going to go with my smaller airbrush. It's more of a model to touchup paint job airbrush. But if you think about it you aren't trying to cover the area like on a car. I have a badger airbrush with a 5 gallon air tank (tank cost me $20 at walmart). I'm going to utilize model paint because I know which color I'm using and it only comes that way. No activator needed just a can of paint thinner to make it easier to spray. Then I'll use an automotive clear coat to be make it a harder clear coat. The smaller air brush will cost anywhere from $30-$50 anywhere. The paint is much cheaper and will do the same thing. Since you have primed it the paint will stick to the surface anyway. They make metal models that it works wonders on and has a really hard paint surface once dried. I still like clear coat makes it look even better. I'll tell you how it comes out but it's going to be a few weeks. I still have to strip the bike down and tear everything off. This is just an idea for you if you want to try it and post on it.

I wouldn't recomend rattle cans. They don't do quite as good of job.