At the risk of asking the obvious question, when peoples’ sweat corrodes or eats away at plastic surfaces like computer keyboards over time, where does the plastic go? Does it get carried around in a coating of plastic on the person’s fingertips?
I’m going to bet a lot of the keyboard material is converted to very fine dust. Some ends upon your fingertips and is wiped off onto the next thing you touch. Other of the dust slips down between the keys along with all the dead skin particles you’re constantly sloughing off.
Thinking of this as a “coating” on your fingers is almost certainly wrong.