Is there any special preparation needed before sanding or polishing?

No, not usually, but removing built up deposits of bitumen/tar with kerosene is a must. If the job is badly stained by red dirt oxidation, We recommend a clean with aluminium brightener/acid to get a clean look in places where an orbital polisher with lamb wool pads are used instead of the grinder and airway wheels. Staining under exhausts can sometimes be removed by acid cleaning, but if it’s badly stained, it usually will need light sanding.