I'm cleaning up an old pair of Weinmann brake levers. Dissolved the remnants of the old liquified lever hoods with mineral spirits, but I'm not sure what do do--if anything-about the threaded plastic cable adjustment fittings that thread on where the (non-aero) cable emerges from the body of the brake lever. They're completely functional, but the part that was exposed to the light has a light chalky layer on it. The part that was not exposed to light is gray. Anyone know a good way to remove what I assume is a thin oxidation layer without damaging the plastic? I can leave them as is, but if there's a good way to clean them I'd kind of like to do it.

I've made Simplex derailer plastic parts with that chalky stuff on them look almost like new by soaking them an Armor All for a couple of days.

Try a bit of Vasileine on them. Car enthusiasts use it to restore sun dried out (greyed) black plastic bumper trim.

Chombi
84 Peugeot PSV

A trick thats worked for me in the past has been to buff the plastic by hand using a long strip of cotton t-shirt and car wax. Similar to polishing alloy hubs, the wrapped cotton strip usually does wonders. Armor All, Vaseline or other liquids that aren't too abrasive would probably work as well.

I have also done this numerous times to chalky, oxidized, black painted mtb bars with excellent results. WD-40 works well on the bars, but I would be hesitant to use it on plastic bits for fear of breaking down the plastic.

Think twice before using Armor All........One thing I learned about Armor All is, with some materials like vinyl/rubber/plastics, you will have to feed the material the stuff constantly once you start, or it will actually start deteriorating quicker than if you didn't use the Armor All on them at all. I noticed this on the cars that I have owned some years ago. Bathed my cars on the stuff and ended up playing a losing game with trim rot on the car and was surprised to find out that the areas I did not use the stuff on ended up lasting longer on the car while the Armor All treated areas just seems to keep deteriorating. I even had the seals fail on the gas filled trunk lid support of one of my cars when a little bit of Armor All got on it when I was putting the stuff on the trunk edge gaskets next to them! Quit using Armor All alltogethe on my later cars and did not have the trim rot/deterioration as the cars I did it on. Many posts around the internet in car sites say the same thing about Armor All too. I dunno, but maybe they might have changed their formulation after all these years after the word got out, but I choose not to ever use the stuff on anything I own these days.
I actually use Pledge furniture polish for cleaning, polishing and preserving the finishes on parts of my car (just too big to Pledge all over), motorcylcle and bicycle. Car collectors have used Pledge for years to prepare their cars for shows and general polishing/preserving of finishes and I have not found anything better and cheaper to use. It cleans, polishes and preserves without buildup.
JMOs

Chombi
84 Peugeot PSV

I used Armor All on a car dash and it destroyed it. Once you spray it on, you better keep it on.

Has anyone ever tried automotive rubbing compound? In small amounts, carefully?

Honda Spray Cleaner & Polish. Won't harm plastic, won't make it slippery, and turns it black again.....

I'm a recovering car nut and I used to use a product called "Back to Black" on plastic trim. I don't recall who makes it, but I think I have a bottle at home.

I've heard the stories about Armor All too, but I don't think that anyone claims that it does any harm to hard plastics. I've been using it since it first hit the market in the early seventies. I don't think I've ever harmed anything with it.

Thanks for the advice. FWIW I tried some Bike Lust silicone bike polish and an old t-shirt with good results--probably about the same stuff as Lemon Pledge.