A few years ago, I found some brush on conductive paint for repairing
PCB's. Perhaps there is something similar available, like a conductive
pen or similar?

Chemtronics makes a keyboard repair kit, but it costs about $20 and only works
so-so. Your best bet is to either buy a new phone or limit yourself to numbers
that don't include 3, or best of all discard the phone so nobody can call you.
(such as "friends" needing to be bailed out of jail).