To be clear, I never said it wasn't a chemical reaction, or that it was. I actually don't know. I only know that the result is similar. Further, I've only seen this particular thing with lacquer finishes, but I don't recall having removed a pick guard from a varnish finish guitar. Poly finishes seem impervious. I have seen what I'm pretty certain are compression dents from mandolin arm rests - the clamp-on type.
One theory with the pick guards and JP armrests are the the adhesive material prevents plasticizers from off gassing leading to the uneven ripples in the finish. This seems reasonable enough, but I'm not a chemical engineer. It may not be entirely chemical, but I don't think it's entirely mechanical either. I just can't say with certainty.
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