If you have access to a good buffer, automotive type you can use automotive buffing compound to buff out many scratches. An automotive electric "polisher" may work but I haven't tried it. The buffer spins faster and has more tork.
If the scratches are really deep, sand then out with 400 wet or dry sandpaper and water and a sanding block. You must use a sanding block and "feather out" the scratches, you don't want grooves. Then compound and buffer. If the 400 sand scratches don't come out easily, sand it again with 600 wet. I have done this on helmets, plastic on my sp500 and plastic on several friends bikes and face shieldes (with 1200 sandpaper and a stationary buffer). You can use the compound by hand but it's a lot more work and you probably don't want to do much sanding with sandpaper if you are going to buff by hand. Wash off the compound and then Armorall. It makes look like new! Hope this helps.