when my airbrush was out of commission, i tried a technique similar to the masking/airbrush technique above, using thinned water color over a Futured windshield.
was heavier than what you were looking for, as i was doing a filthy desert vehicle,
but if you went thinner, and a lighter application, it might work.
nice thing abt watercolors is they won't stick to the future too hard.
so if it comes out badly, you should be able to wipe it clear.