I would never apply anything to my speaker surrounds. It can do more damage than good. As long as you keep your speakers from exposure to any of the known things that can cause degradation of the surrounds, the surrounds should last a lifetime. The things to avoid are UV (from sunlight and fluorescent lighting), Ozone (laser printers are big culprits), VOCs (from paint, household cleaners, furniture polish, metal cleaners and of course cigarette smoke) and excessive heat and humidity.
I purchased my KEF Calinda speakers in 1979 and they are still going strong in my third system. The surrounds are rubber and still look like the day I purchased them after 40 years. My KEF 104/2 speakers were purchased in 1991. They are the biwire version and have foam surrounds and foam donuts on the woofers. After 28 years they still look like new and show no signs of degradation.
I purchased my KEF Calinda speakers in 1979 and they are still going strong in my third system. The surrounds are rubber and still look like the day I purchased them after 40 years. My KEF 104/2 speakers were purchased in 1991. They are the biwire version and have foam surrounds and foam donuts on the woofers. After 28 years they still look like new and show no signs of degradation.