"WHITE METAL FITTINGS
Clean up any flash, or mould lines on cast metal fittings, using fine files or fine sandpaper. Surfaces, which are to be flat should either be filed or rubbed across a sheet of Wet and Dry paper glued to a surface (use a contact adhesive). Apply a coat of primer or matt white paint before final painting.
Spraying and painting small fittings like railings and vents can be a bit of a trial. Drill a piece of flat wood at intervals to take the parts, then cover in double-sided sticky tape; push the parts into place to be held firm by both tape and hole. You can then spray away to your hearts content and they will stay put."
(With thanks to Model Slipway's Hints and Tips web page).
In addition, you can give the part a good scrub with a fine brass-wire brush (e.g. a suede brush) to polish it up. I then give 'em a wipe over with white spirit, IPA or meths before applying the primer, to make sure there's no black residue left to spoil the paint job.
Works for me - suit yourself.
FLJ