Read Marv's answer. Doesn't make sense to coat the felt part in the center of a speaker. It won't be brittle anyway.

Norm

:Vinny
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: In some cases I've coated the entire cone. Since rubber cement remains flexible doesn't effect sound. Don't use anything that hardens. That will cause a loss of low frequency response.
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:Norm
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::Norm, thank you, do you also coat the middle section and does it affect the sound?
::Vinny A.
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:::Hi Vinny
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::: I use Carters rubber cement with good results. Softens a brittle cone and has held up well over the years.
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:::Norm
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::::Greetings to all,
::::Has anyone heard of or does anyone use rubber cement to coat speakers? Is there a proven revitalizing method for speaker material?
::::Thank you,
::::Vinny A.