W Metals can be directly painted on plastic, without primer.
You can also use them as a primer, before the actual painting. And / or as a metallic preshading.
A glossy, smooth and even surface is preferred to obtain the maximum brightness of W Metal. You can eventuallly polish a bit the plastic with a smooth cloth, before painting.
Here how to use them by airbrush :
Both for airbrush or brush, you can use them without without dilution - this way better when painting by airbrush - or dilution with plain water.
If you need or if you like to use of a primer, it's advisable to use True Earth's SLICK PRIMER or WHITE PRIMER.
Both for panel lines and for more or less marked effects of wear, you can combine the basic painting with W Metals together with METALINER products.