I bought some cerium oxide (brown powder) quite a while ago on Amazon, but I never worked up the courage to use it on my Nauticam WWL-1. I had noticed that whenever I did not use an air blower to dry the lens, it developed stains that were impossible to remove with regular cleaning methods. After reading comments saying that the anti reflective coating is not on the outer surface, I decided to give cerium oxide a try. The results were very good. I mixed the cerium oxide with water to make a paste. If the tap water in your area is very hard, it is better to use distilled water. I used two parts of water for every part of cerium oxide. The more diluted the mixture, the less abrasive it will be. I recommend polishing it by hand with a microfiber cloth. Using circular motions and making several passes removed all the stains from the glass surface. Honestly, it did not take much effort, and the result was excellent. The glass now looks brand new.
