What you'll need:
3M Lens & Hard Plastic Cleaner (P/N 39017)
3M Plastic Polish (P/N 39010)
A couple of clean cloths
Recommended:
Some form of mechanical polishing tool
I cleaned and polished these by hand, but with a good tool, you can get a better finish than I did.
First, start with removing and cleaning the part. In my case, I removed the cowl covers and took them to the sink. Using some dish soap, I cleaned them down and sprayed all the leaves, bugs, and crap out of the mesh. Once you have them nice and clean, you should have something that looks like this:
You can see how weathered they look in these shots.
First, take your cleaning compound and put a nickel or so sized dab on one of your cloths.
Then work that into a section of the plastic. Use a circular motion and keep working the same small area until you're no longer seeing streaks of the cleaner. Once you've done that, just put another dab and start working out a new section. You can use clean parts of the rag to wipe away excess cleaner. After a while of rubbing, you'll end up with the first stage done. It'll have a slight haze to it, but it won't be nearly as bad as it once was.
I found it best to take a clean part of the rag, and try to wipe off as much of the haze as possible. I don't think it's necessary, but it was just something I liked to do. Also, take a paper towel and start wiping out the excess inside any holes you have. Since these cowl covers have vents, I had to clean excess material out of them as well as the holes for the seal's clips.
Take your next clean cloth and dab on some polish and repeat the process of rubbing it in. Just keep rubbing until you no longer see any streaks of the compound.
Not perfect, but a nice improvement. I buffed these down with a microfiber afterwards to remove any left over residue. Any piece you do can always benefit from additional polishing. Just remember these compounds do not work on textured plastics, only flat plastics. Test these products on an unseen side of whatever you intent to restore to make sure they won't damage the part.
