I dilute my enamels with laquer thinner. For airbrush use, it should be the consistancy of milk. Just add the thinner a few drops at a time and give the bottle a good shake. With laquer thinner if you add too much it isn't a big deal.
Just leave the jar open for a few minutes and it will evaporate off. The laquer thinner won't damage the plastic on your model, either but, the paint will dry more quickly and bond tighter to the plastic. Do some test spraying on a piece of cardboard or a plastic "FOR SALE" sign. Also, adequate ventilation is critical.
Ray