If you’ve ever made anything with ABS plastic, you know that it can get discolored over time thanks to sun exposure — this especially applies to stormtrooper armor. It’s white, so the yellowing from UV really shows up and on top of that, stormtroopers often participate in outdoor events if they’re part of the 501st Legion and are in direct sunlight. Instructables user Lovelikeatruck has dealt with this problem; he pulled his armor from storage after a few months and discovered the helmet had turned an unappealing shade of yellow. He figured out how to restore it to the more pristine white color and put together a how-to on Instructables.
The primary material is homemade Retrobright made from hydrogen peroxide, xantham gum, glyerin, and Oxyclean. After he mixed the material he said:
I applied the Retr0bright with a paintbrush, making sure to get a fairly even coat over the entire surface. It should not affect any paint or the vinyl decals, so I painted right over them. I then placed the helmet under my UV light. You can also do this outside on a sunny day. However, you want to be careful that your helmet doesn’t get too hot in the sun or it will damage the plastic. I decided to play it safe and use a reptile light, it also gave me the ability to leave my helmet unattended without worrying about something happening to it (like a dog thinking it looks fun to play with).
I left it under the light for about 12 hours, rotating it every couple of hours.
Read more at Instructables.