Germanium diodes with their paint markings removed91% isopropyl alcohol will weaken the bond between the glass and the paint. A little rubbing and the colored rings will come off.
I have a few dozen diodes soaking in a bottle of 91% isopropyl for over a week now. The alcohol is now a light pink color but the bands are still intact on the diodes. So the IPA will not completely dissolve the paint, just loosen it.
I suspect some other types of solvent will completely dissolve the paint markings without having to scrub them off. Will experiment with acetone and whatever else the hardware store has.
An original red banded diode and two bare germanium diodes. 1n4148 diode for comparison.