I'm pretty sure Johnny Sawn "went through the grades" on wet and dry to get good A-pillar trim. I expect he started high though - 1200 to 1500. If you went as low as say 800 you might scuff the surface to the point where you can't get it back to smooth again. I'd also suggest using it wet rather than dry and see what happens.
You could always PM him through the forum to ask him what grades he used to do it.
Whatever you do, you might have to do it to both the old one and to a lesser extend the new one, so they looked the same when finished.
Mildly abrasive substances could also be used to get the perfect surface finish - Autoglym super resin polish, toothpaste (believe it or not) or Brasso all have the right level of abrasive content to polish plastic such as this. I expect toothpaste will probably leave you with a dull satin finish and the likes of resin polish will start to give the plastic a shine. Same as using wet and dry I suppose - stop at the finest abrasive particle size where you get the look you want. They were never "shiny" when new - repeated washing over decades tends to make them very shiny. They were a satin ("eggshell") finish.
I expect if they're really bad you could remove them and spray them (rough the surfaces 800 grit, adhesion promoter, plastic primer, and a top-coat of satin black) but it would have to be a perfectly flat and smooth final spray so they looked original. Probably a job for a body shop or vehicle restoration company used to do this kind of thing - a spray gun will probably get you a super-smooth finish.
If your "old one" is badly pitted, this might be the way to go - you could fill the surface imperfections with plastic filler before prepping and then putting on each layer of paint to both trims, so they looked the same.














