Even though these were white to begin with, they were a PITA to prep. Only because someone painted them yellow and they sat outside in my yard for 2 years. Sandpaper or powerwashing would not get the yellow paint off. Had to use stripper for plastic bumper from O'Reilly's. If they weren't painted, I would just cleaned them up good with Simple Green and the red scuff pad, dry and clean with contact cleaner and paint. No primer necessary per Krylon's instructions.
I was on the fence to paint these black with a similar scheme in yellow/green, Monster type theme, but considered the condition of these. If they chipped, it would really look like crap with the white underneath. Now, I probably won't be noticeable too much if they chip. Going to try to do black ones next.