I think you may have hit the nail on the head with the sweaty hands thing.
I use the MX Ergo (and various other Logitech trackballs since probably the late 90s). In my experience, that 'sticky' surface which develops on the rubber is crud from your hands. It can happen with Sky remote controls and any other rubberised device that is handled (especially held) for lengthy periods. If left too long, it becomes a brown-ish, semi-glossy coat!
I find it worse in summer when you're likely to be sweating more.
I wipe mine down with a tissue and (of all things) Autoglym every couple of weeks. Usually when I take the trackball out to remove any crud on the support bearings
Biting the hand that feeds IT
