As I said in the first message, I tried carbon on paper (pencil) but the result is not very linear, I think the result with the painting will be similar. I will, indeed, probably have to isolate the part in contact with the finger to avoid disturbances.This is also why the capacitive sensor could be a good idea too.
my problem is already solved (probably), I just need a resistance of more than 1K, long, flexible and linear. I will try with a ceramic resistance when I have one. just the idea of the capacitive sensor interest me too.