I know this is an old post - but here is my experience using similar hair bleaching creams to reverse the fire retardants from coming to the surface. (from what my research found as the cause and multiple discussions about this on First Imperial Stormtrooper forums)
Interesting thing is, when looking at parts like the shoulder bells, the INSIDE of the bells are white, the OUTSIDE turn yellow - or IMO the side that is stretched when vac forming. Does this have anything to do with the cause of yellowing? 🤔
I have done this twice with my TK with the first time back in 2019, it brought it back but not completely to the original white. My armor is kept in a bin, in the garage away from any direct sunlight. I religiously used marine grade UV plastic protectants over the armor before and after every troop.
Like everyone here states, the process appeared to not cause any physical damage to the plastic. Unfortunately, the armor started yellowing again but not like overnight but over time. The pic below shows what it looked like in 2021 after the doing the process the first time and only after 4 troops (due to Covid). The after doing this hydrogen peroxide process once again is on the right. I used a UV spot light this time and sunlight. (the sun is intense here at 4,500 feet above sea level in the Sierra Nevada Mountains)
So my findings - yes using the hydrogen peroxide will reversed the fire retardant yellowing but from what I researched, it doesn't remove the chemicals but pushes them back into the plastic. I could be wrong. The problem is in 2023 my last troop at Star Wars Day for a local baseball minor league event, my armor was already showing yellowing when compare to another TK, I was standing next to. This was only three troops since the 2nd hydrogen peroxide treatment below.
So IMO - using this process only increased the rate at which yellowing occurs post treatment. Painting may be the only permanent recourse. I'm not sure if clear coating will help over new armor. Interesting thing, my ESB hand plates were made from vac formed PVC and it is still bright white - 10 years later!
YMMV - and like the veteran TK members say, "All TKs eventually become TDs" for this very unavoidable reason. 😊
