Vortex Sound Research

"Electric paint" is often used for drawing basic circuits, and for repairs like renewing contacts on old remote control buttons. It's also ideal for connecting solder jumpers on modules. Those solder jumpers are often tiny, so a bit scary with a solder iron.

Avoid the solder fear/hassle and use some conductive "electric" paint, dab it on with a toothpick, and you're done.

To undo, just cut between the jumper contacts with a sharp blade.

This stuff.