I have a genuine Cossie grille that's been sat in my shed since last year, so I might as well paint it up and put it on the cabby. It's even got the black mesh bit with it

Anyway, it's currently unpainted white plastic. If I go down halfrauds for the spray, what do I need to do to ensure it paints alright? Do I have to rub it down with some fine sandpaper? Or do I just spray it and then laquer it?