Originally Posted by Frank Reiner

Is not the "rubbing up against it" action one of making contact with a resistance? When the wiper is moved along the resistor, the resistance value changes with the position of the wiper.
If an insulating material (varnish or the like) is applied to the contact surface, there will no longer be contact and conductivity between the wiper and the resistor.

Originally Posted by 97 SL320

The black areas are conductive and is what gives the sensor variable resistance so insulating paint won't work.

A paint kit that repairs heated rear windows is a possibility however we don't know the resistance per area of the old and new paint.

Have a look at the center terminal, go to the right and down a slight bit. You will see a thin white line. Look further to the right and you will see more lines. These are laser cut lines to adjust the sensor linearity and calibration.