I wouldn't use sandpaper unless the scratches are really deep. If they don't catch your fingernail you can use any number of plastic/acrylic polishes including Novus, Meguiar's, Zaino, or any of the other quality plastic cleaners/polishes. Always start out using the least abrasive polish and work your way up if won't completely remove the scratches. Also use a good quality microfiber or 100% cotton towel to apply and remove.
Edited to add: 3M and Plexus also make very good plastic/acrylic polishes and scratch removers, and most all the brands are available online at car detailing places.