I bought my current piano (CVP 96) in defective condition.
Randomly some keys did not respond and others randomly responded at full velocity. (So I got it very very cheap;-)
I have also seen some other keyboards from inside, but not a Fatar.
In all cases the electrical contacts where capsulated in some rubber, so they cannot contaminate.
(Nonwithstanding a little bug or somthing like this can creep inside ;-)
There are two contacts: an upper and an lower, the timing difference between them makes the velocity measure.
If the upper contact is broken then the keys always respond with maximum velocity. If the lower is broken they dont respond at all.
In the case of my CVP 96 it turned out, that the contacts where made from a carbon material that is hard as glass.
Over time these had lost their contact to the PCB traces or gave random contact.
I cleaned all traces and contacts and reconnected them with conductive silver color. A lot of work!
However since then it is 100% functional again...
HTH,
Peter