True, but one does not need to moisten anything at all. Grind a few granulae of old radium paint [under some inert solvent, like alcohol, so it does not dust], let the alcohol dry. Make some epoxy, smear it on one side of the sights glass inserts. dip each insert in the radium paint dust - it would stick to epoxy. allow to dry, and then cement the insert with the paint glued to one side into the sights cavities, so the paint is not even exposed, but is covered with glass at all times.
Ottawa, IL, a bit west of me was once home to radium dial painting companies.
Local legend has it that some of the women painted their nails with the stuff for a fun night out. They did in fact moisten the brushes in their mouth.