Stuart,
I am not sure what you consider structural, but with the technology
available today in adhesives, I disagree with you on this one.
I have used the body panel repair adhesive in many applications, and I
have not had one comeback, from one customer, ever. Period.
If the body panel adhesive was a poor idea, and it was prone to failure
on a regular basis,
 I would suspect that 3-M, and other manufacturers of this type of
adhesive would be having one hell of a rash of libel suit's piling up on
their desk's, if the stuff didn't work.
Like I stated before, wrong application, wrong preparation, is more than
likely the culprit here, I don't care what the shop's credentials are,
human error could be part of the failure factor.
And above all else remember this----
The Titanic wasn't suppose to sink either.
But a slight error in design took care of it.
People screw up !!
Shit happen's !!
And I would venture to say that this is the sole reason this product
failed in your case.

Have a beautiful Day !!

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Terry 85 GL