Stiffness (compliance) of a surround will partially determine the speaker's primary resonance (Fs), so it stands that altering the surround will possibly shift its resonance. It's my understanding that Armorall is a monomer dispersed in an aqueous (H2O is of course Earth's most prevalent solvent) solution, and its action is to deposit a one-molecule thick "preserving" layer on vinyl, and possibly butyl, as well. The fact that you can see it on butyl surrounds after it dries lends me to think that it's helping to seal them as it does very well on vinyl and rubber auto door seals and trim. Trouble is: how do you treat the backside?! Maybe the relative effects of spider-hardening vs surround-stiffening are shifted as a function of ratio of cone/voicecoil diameters, too: I can imagine that the quality of the surround of a 12" woofer is relatively more important than the aging of its spider, whereas the reverse might be true of a 4" midrange with a relatively stiff surround to begin with. Just conjecture, though. In the past I have run into high-tolerance voicecoil-gap drivers that scraped after a few years, probably due to spider "sagging". Running the woofer upsidedown for a while usually remedied the problem. So gravity figures in this, too!
