Whats a good airbrush coat that would stick to metal?

 

Well, you can shoot any paint you would use on an automobile - if you use a proper safety mask rated for organic solvent protection.  The stuff contains isocyanates which are very poisonous.  Or you can use Createx Auto-Air water based acrylic paint.  In and of itself, it is not waterproof and has to be coated with a durable waterproof topcoat.  Depending on the object to be painted, It would also help to use a self-etching metal primer before you shoot your color coat.