Coated woofer - why?

Btw this was the way Sonus Faber did the same FS (resonant frequency) lowering and stiffening of their bass cones on their stuningly magical sounding Extremas.

https://images.app.goo.gl/w9frGfkLN9dfrJoJ6

Cheers George

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.

thosb, I think as simmonmoon suggests it is time to replace those drivers.
Modifying drivers in the context of a speaker system without ample test equipment is a sketchy thing to do. As an example the coating added mass to the cone which changes the characteristics of the driver and then it's relationship to the crossover. God knows what that did to the speaker.
The LS 3/5A is a magic little loudspeaker. Why anyone would want to mess with it is beyond me.

Seas is coating some drivers with graphene now. Not sure what the benefit is, or if its just part of the "put graphene on everything" rage going on right now.