nice work
With the grainy paint finish you could also add a touch extra bit of thinner or some sort of Paint Retarder (Tamiya and Gunze both have one for their range of acrylics - don't know about enamels or lacquers). This would help with keep the paint wet - but has a longer drying time than normal.
Are you test spraying on a piece of paper/scrape plastic/wood before hand ?
The paint should gone on wet or have a gloss appearance. Test spraying may help identify paint-to-thinner ratio problems that could causing a grainy effect when applied.
Track down (I found this at a library) some DVD's/Videos on artists using airbrushes.
have fun
A tip i've read many times, (but never done myself) once you've masked, airbrush on the under colour again or a clear coat of some sort - this helps seal the masking edges.