Dry film coating thickness gauge.
https://diqn32j8nouaz.cloudfront.net...e_fullsize.jpg
Similar tool in use:
https://www.youtube.com/watch?v=m10JoYviDX8
Previously:
https://www.homemadetools.net/forum/...857#post163940
https://www.homemadetools.net/forum/...217#post130586
https://www.homemadetools.net/forum/...582#post197409
https://www.homemadetools.net/forum/...634#post193481
https://www.homemadetools.net/forum/...149#post191687
