Researched and written by Christopher Pilant.
Occasionally, we find ourselves in the position of having to deal with an irritating problem: small amounts of silicone grease or Fomblin, used on caseback threads, working their way onto the inside of the crystal. The task of removing it is frustrating, at best. While ultrasonic cleaners and Lenspens are reliable solutions, commercial lens cleaner and disposable, lint-free paper towels proved to be less so.
As we began to wipe the crystal, using paper towels moistened with lens cleaner, we noticed that as the cleaner began to evaporate, fresh smudges appeared. This caused us to chase our tails for a bit, cleaning old smudges for new ones to materialize.
After some experimentation, we concluded that the commercial cleaner acts as a solvent for the binder in the lint-free cloths, and redeposits it as fresh, greasy smears on the surface of the crystal. Which must then be buffed away with a fresh cloth.
So, please learn from our mistakes. Save yourself the irritation (do the environment a favor) and use a clean, washable microfibre cloth.