you can still work with yellowing stormtrooper armor. since it's vacformed it won't need as much prep time, but i'd still suggest sanding it lightly with 400 grit sandpaper. after that give it a coat or two of rustoleum automotive primer. then wet sand with 800 or even 1000 grit (personally i would just stick to 800), don't sand through the primer, just make sure the surface is smooth as a baby's butt. once you have your perfect surface, time for some high gloss white, rustoleum preferred, 3-4 light coats, then 1-2 medium coats. to finish it all off get some high gloss clear coat and give it a couple coats with that. you'll be parade dress shiny in your armor then.