Lens makers have used rubber to make the focusing and zoom rings. Rubber rings give a better grip and control of the lens than metal rings with grooves. However, they are not as durable as the metal rings.
One common problem of rubber rings are the oxidation and deterioration of them. Almost everything in contact with the oxygen in the air would oxidise. The contact with sweat, air and sunlight (uv light) speeds up deterioration.
I found my Canon lenses rubber grips turns white or grey after a couple of years of use. Some of them were rarely used. Most of their time was spent inside the humidity controlled cabinet. Later, the whitened rubber turned hard and sleek.
There were many photographers faced the same problems. I searched google and found many improvised ways to remove the whiteness. Some were beyond my imagination, like using vegetable oil. I wondered if the vegetable oil would attract cockroaches.
I had tried almost all of the methods I could find from google. I learned a lot about how not treat the lenses. Organic solvents like rubbing alcohol would erode the lens rubber parts. Toothbrushes could scratch the rubber surface. Tire wax would make the lens rubber dangerously slippery.
There wasn't a reliable and proper way to solve the problem.
I experimented on various ways to repair and restore the oxidised rubber rings to make them black and bright again. I recorded and improved the solutions throughout the process.
Eventually, I created the Delicate Lens Rubber Cleaner. It's a solution to cure whitened lens rubber rings. Check out the ingredients and you would be amazed how they could come together to cure lenses.