I have this experience of inspecting some parlyene coated board and like you mentioned is impossible to see the difference. The surface of a coated board appear to be slightly matte or duller compare to uncoated board. Again, it take experience to differentiate what is matte and what is not. I even asked the vendor how to differentiate a coated board and they too of no help. 

Unfortunately, my reply does not seem to of any help to you.

Regards,
Wee Mei

--------------------------------------------

Does anyone in this group have experience with variations in the appearance
of parylene coating?  Our vendor is applying parylene on a small PCB.  Most of
the boards still exhibit the glossy finish of the original board mask but we see
some units that have a distinctly dull or matte finish.  Sometimes it is limited
to one end or even a corner of the board.  The vendor reports that this
reflects a slightly thicker deposition of the parylene coating and is not a cause
for concern.  Can anyone confirm this, or are there other ideas that might
explain this condition?  Thanks for your help.

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