depending on how bad they are, some use wet and dry and lots of water on them. starting with a mildly coarse girt/grade and work to a finer grade in a different direction to the previous grade until it is how you want.
when i do paint i cut back the rough stuff (usually primer/filler) with 400,
i’d start with about an 800 for any mild scratches and work up to a 2500 to finish off with.
but on plastics that are just faded maybe start at about 1200 and work to 2500. then hit her with some spray silicone . 