Yes, the potentiometers are coal based and could be worn but in most cases it's just dust and dirt on the coal tracks and adjacent contact tracks. Coal tracks are treated with contact cleaner, leave it to soak for a couple of minutes and then wipe dry using cotton buds. The contact tracks needs a good rub with a glasshair brush to make them like new again, they are merely a metal bar. The sliding (bridge) part also needs a gentle cleaning with contact cleaner. Don't leave contact cleaner inside the potentiometer. Dry and clean is good.Originally, the sliders were lubricated using some kind of grease, probably silicone based, you can clean and add new silicon grease if you like (non-hardening type obviously). The potentiometers can be opened fairly easily, comes apart as two plastic shells and a tiny drop of superglue each end of the plastic shells will hold it together again. Furthermore, the holding brackets will also support once mounted.
Martin