I would recomend a decent plastic primer before applying any other paint. Not all paints stick to plastic but most paints stick to the primer. I have seen items painted with nailpolish, sealed with a clear coat of paint.
About dry brushing: I usually paint the whole thing metalic first, then apply black or rust colors. Buffing the black off, leaves a natural wear of the metal. Metal usually rusts in the corners where there is no contact with other items. So that's where you want the black/rust to stay. Here's one of my creations I've painted with this technique.