The best way for painting brass is making sure that there is no grease (fingers
) anywhere on the metal. I usually put my brass models in a soda bath for a while, but that doesn't work for a plastic model with PE details. My avatar shows a brass railway model, but that's not a good comparison as it is a full metal model.
But to let any paint adhere better to the metal I spray brass always with a very thin coat of wash primer or etch primer. It adheres to the metal because of the slight etching quality of the primer. Car primers (Motip for instance) usually do their work as well, but I would never use that for a small model directly out of the spraying can.
The etch primer I use is from Phildie and leaves an extremely thin almost transparant coat without covering any details. On top of this primer you can use any type of paint and it will stay there.
Both types of primer (Phildie and MOTIP) won't cause any problem on the plastic parts of a polystyrene model.
regards,
Ivo