John has some good points there, and priming to give the paint some tooth to adhere to is excellent advice.
Which Golden's colors are you using as they make two that can be used for airbrushing.
Are you using the Golden Airbrush Acrylics or the Golden Fluid Acrylics?
I know that a very gifted airbrush artist that I used to know and who recently passed away used Golden's exclusively for her automotive and motorcycle airbrush artwork, so they obviously adhere to metal well so plastic should be very similar. If you care to see what she did with Golden's check her web page that her husband still maintains.
http://www.ginag.com/
There is a Tech Page at Golden's web site which may also be of help here:
http://www.goldenpaints.com/technicaldata/techintro.php
Mike
“Wisdom is the right use of knowledge. To know is not
to be wise. Many men know a great deal, and are all the greater fools
for it. There is no fool so great a fool as a knowing fool. But to know
how to use knowledge is to have wisdom. " Charles Spurgeon