Okay, so you've got a vintage computer or whatever and the case has gone all yellow-brown? Thought you were doomed, 'cos, hey, the plastic is deteriorating?
Maybe not. The "browns with age" thing is a chemical reaction, all right, from fire-retardant chemicals in the plastic (likely containing bromine, if I understand what I've read so far) and the effect can be reversed! Not totally, but it'll take an old smoker's-lung computer case a long ways back towards the original hue. Have a look here (retrocomputer hobbyist BBS).
Dabba Oxy-Clean, bitta 12% hydrogen peroxide, UV light, good ventilation, use eye protection, do not ingest mixture, keep away from kids, pets and Leftists, yadda-yadda. I have not tried this, just sharin' what stumbled across.
It's entirely possible this trick may have other applications. Experiment at your own risk.
One Evening On Kansas II
1 week ago