Hah, it's funny this topic came up.. I was about to create one similar.
I'm actually in the middle of a project to revamp some flash carts, which I'm trying to rehouse. I'm doing away with the plain, boring looking black shells and am swapping the PCBs into some clear/translucent Lexan cart shells that I finally obtained. I've done some reading and here's what I learned so far.
You need to know what type of plastic you're working with. For example, most cart shells are made from polycarbonate/Lexan, especially translucent ones, but solid color shells can be ABS too. Lexan is stronger and shinier by nature but it's harder to polish than ABS. Vapor polishing seems to be the way to go but it's practically impossible for a home type project, due to it being a dangerous, toxic process. Nobody in their right mind is going to send off some old cartridge shells or consoles to a company to have them do it either. Though flame polishing is another popular method, it's not the best method to use with Lexan because it's a very touchy process and it's extremely easy to screw it up by burning the plastic. Just 10-20 degrees too much, heat wise is all it takes to go from perfectly polished to toasted. So good 'ol elbow grease will have to do.
I too just picked up the Novus polishing system. If you used Novus #3 heavy scratch polish on anything translucent, it says on the back of the bottle Not to use it on polycarbonate.. Which might be the reason it hazed up. Novus #2 and #1 are supposedly safe on polycarbonate though. ABS you're good to go on the 1 through 3 polishes. You're supposed to start with #3, or #2, depending on the severity of the scratches, then finish it off with #1. Be sure to completely wipe off the polish and use a fresh cloth, etc before switching polishes. Washyyour parts with dish soap and water. Do not use isopropyl alcohol to clean those types of plastics and don't use any petroleum type products (like WD40) on those types of plastics either, as they'll both weaken the plastic and could cause crazing sooner or later, then you'll really be screwed.
I did a little polishing by hand today and it helped but I think I'm going to bust out the Dremel with a polishing cone and wheel on it tomorrow, using it at a low speed of course. The one clear intellivision cart shell has some pretty nasty scratches due to some mold flaws, so I might have to wet sand those scratches, starting with 600 grit, working my way on up to 3000g, then start with Novus #2, then finish with #1. We'll see how it goes. It sucks I couldn't just start off with #3 and I still might try it, but it'll be on a small area. However, I hate to take a chance on screwing these shells up since they were very hard to come by in the first place! Lol
Non-textured plastic is one thing but as someone else already mentioned, be really careful on any textured plastic.. Don't over-do it. Anyways, good luck with your plastic restorations!