A little rough around the edges...
So you want to do a helmet project. Really? My advice... turn around and walk out now while you still have time and money. Okay, so I'm kidding. Helmet projects are really not all that difficult when it comes right down to it, depending on the issues involved.
Some helmets just need a good buffing...
That's not a lighting trick. You can use hand buffers or a good professional buffing machine. The key is cleaning the helmet then using a high speed buffing machine with appropriate abrasive compound to buff and polish the helmet. Depending on the machine and your personal level of perfectionism, this process could take 5 to 20 minutes. Remember you're dealing with plastics. They are affected by heat. Heat is bad for them if you hyperfocus on one flaw at a time. Keep changing angles during the buffing cycle. Even a beginner can get some pretty amazing results.
This is highly effective on clear shell helmets but can be equally as effective on normal, colored plastic helmets.
Patience is the key... Good luck!