how to remove scratch from black plastic (rear)
 

I have scratched the black plastic just over the rear exhaust on the back, can you use wet sand paper to smooth this down to take the scratch out or does it take the shiny finish off the plastic?

Cheers

99% of the time the repair will stand out more than the original problem. I really think 100% of the time a repaired spot on black will show.

You can try using some citrus-based cleaner (like Goo Gone).. it will melt the plastic with the goal of making it blend with its surrounding area better. Just a few seconds will do (maybe 10 seconds for a fine scratch?) and then apply a rubber/vinyl protectant like 303 Aerospace (Armor All or the like will do) to hide it even more.

To fill in deeper scratches, you can use Mr. Clean Magic Eraser. Be aware that some of the Eraser's white compound may discolor the rubber. I would use the citrus to take some of this off and then coat with the protectant. This way should be safer than using sandpaper.

I've done this to shoe scratch marks on the hard plastic panels close to the doors that ignorant passengers always step on. The citrus will remove some of the naturally rough texture on the plastic but the end result works. Hope this helps.

Replace the shroud if it bothers you that much. It can't be too expensive. To answer your question, no, you can't sand plastic.

You technically can with like 2500 grit but you'd have to do it to the whole plastic part to make it even which is more than what it's worth and the potential of screwing it up.

If there's a scratch on trim plastic at work (I'm a detailer), we use tire shine to hide it. If it's too deep that tire shine will not mask it's appearance, then replacing the trim is the only way to go. Sanding it will only cause a texture variance that will be noticeable and it may even turn the plastic gray or white in places.