W Metals can be directly painted on plastic, without primer. Here how to use them by airbrush :
a first thin and almost invisible layer of W Metal at a high pressure
reduce the pressure in a range between 14 and 22 psi depending on your airbrush, and saturate the paint with long parallel passes
Best results are obtained without thinning, or with a minimal thinning with simple water.
By brush , it's preferable to dilute them slightly, with only water.