HP CArd Readers - Longevity

[Kees Bouw]

Very interesting stuff, the ageing of plastics. Some plastic discolours to brown, some plastic disintegrates like the magnetic card drive rollers, some plastics become very hard like the paper transport rollers of a HP-97 printer, and recently I have had HP-67 keys become very ugly after cleaning with warm water and soap, showing many white cracks where the keys have been exposed to light (see attached pictures). Right now I am having difficulty cleaning the case of a HP-67. On the areas where it has been held there is white tarnish, maybe acid from sweat or skin fat has affected the plastic if such a thing is possible. I have not yet found the right means to get this off.
I would welcome a "plastic historian" to explain these phenomena along with some advice how to treat the various kinds of ancient plastic.