Bifocal scuba lens with hard water stains
The scuba lens with hard water stains

Did you know hard water stains can accumulate on and damage the lenses of your scuba mask?

I wear a Gull brand scuba mask, with prescription bifocal lenses. After my prescription changed, I ordered a new mask from Gull, and used the old one as a backup.

I’m diligent about rinsing my gear after each dive, including my backup mask, which sits in a thigh pocket on my wetsuit.

On a recent dive, I used the backup mask and was surprised that the lenses were unclear. What I suspected to be dirt turned out to be hard water stains. While I made a point of rinsing the mask, I had neglected to wipe the lenses afterward. That allowed hard water stains to accumulate on the outside of the lenses.

Hard water solutions

After consulting the internet for suggestions, I soaked both lenses in vinegar, then hydrogen peroxide and then rubbed the stains with a paste of baking soda and water. I tried a glass-safe household cleaner (Jif/Vim) as well as table salt on a lemon wedge. None of this worked.

I emailed Gull in Japan, who suggested contacting an optician shop. I visited three shops here in Hong Kong, with no success.

A second internet search recommended using cerium oxide (CeO2, aka optician’s rouge). Since the lenses were nearly useless in their current condition, I didn’t have much to lose.

Cerium oxide to the rescue

After unsuccessfully trying to buy cerium oxide in Kowloon’s hardware district, I ordered an 8-ounce package of Gordon Glass optical grade cerium oxide polishing compound from Amazon (US$30).

I started by making a slurry of two parts cerium oxide to one part distilled water, and polishing one lens with a microfiber eyeglass cloth. After about five minutes of polishing in a circular motion with light pressure, the stains began to disappear.

I increased the cerium oxide/water ratio to 4:1, switched to a soft plastic pot scrubber and applied a little bit more pressure. After about 10 minutes polishing each lens, the hard water stains were gone. A few scratches remained, but not enough to ruin my view.

Scuba lens after polishing with cerium oxide. The hard water stains are gone
After polishing with cerium oxide, the hard water stains are gone

I rinsed the lenses under hot water and scrubbed them with an old toothbrush to ensure all the polishing compound was removed.

Following safety instructions on the internet, I wore rubber gloves, eye protection and an apron. Cerium oxide is a very fine powder, which I mixed into a paste outdoors to ensure sufficient ventilation. In total, I used less than an ounce of the polishing compound.

Christopher Dillon chaired the Hong Kong–based South China Diving Club from 2018 to 2020.