I tried painting a bicycle frame with an airbrush gun and found it inadequate. Not only would it take ages, it would be very hard to create an even paintjob (it *is* very effective on paint use, though )
Its a bit like painting a sheet of paper one color with a ballpen.

However: There *is* an airbrush gun from Iwata which is a miniature HVLP gun, the Iwata Eclipse G3/G5



They describe it as "Unique “Pistol Grip” airbrush-guns, the Eclipse G-Series look like miniature spray guns, but spray and perform like a large airbrush."
http://www.iwata-medea.com/products/...es/eclipse/g3/

The best part is that it requires such a small ammount of air that allegedly, it can be used with an airbrush compressor. This is nice if you already own an airbrush compressor or, like me, can't run a big one for noise reasons (if that is not a concern for you, i guess using a "real" HVLP gun is the way to go)

I heard of people specializing in things like motorcycle helmets or tanks who use it for the base paint layer.

Downside: It is insanely expensive. But it might just be the thing for those small diameter tubes we are dealing with and for which the usual HVLP paintguns are just oversize.