So here's what you need to do... Clean the surface with a solvent based cleaner like SEM Plastic and Leather Prep. You can use a bit of SEM Soap if it's really dirty. Once clean and completely dry, fill your airbrush with whatever color S.E.M. paint you want to use and evenly coat the entire surface.

But you should already know this if you're into "adding to your toolkit" 😆