Since most plastic is glossy, sanding is then an essential step. Lightly sand all surfaces with 180 to 220-grit sandpaper. Since plastic is soft, you can do the sanding by hand. However, if you have large areas to sand, you may want to use a random orbital sander.
Likewise, how do you wet sand scratches on plastic?
Soak a piece of 800-grit wet/dry sandpaper in water for several minutes until saturated. Use a gentle, circular motion to sand the area, making sure not to apply too much pressure or you could introduce new scratches. Rinse between passes and re-wet the sandpaper as needed throughout the process.