ScotchGard is used for waterproofing speaker cones that may be exposed to water and is not supposed to hurt the output. I have never seen this in person but that might have the same appearance you are describing.
Coated woofer - why?
An old (20+ years) speaker with multiple previous owners has a coated woofer - why would this mod be made? What are the implications for SQ? Speaker is a LS35A type with original KEF B110 woofer.