You sometimes find resin on very old PCB’s, like PCB’s from tube radio’s and tv’s. Often times these work without a hitch unaffected by the resin.
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… Untill you clean them, then it’s a crap shoot 
I was looking for something else, and came across this bit, which could be of interest here.
“I had this mini‑oscilloscope in my rack for more than a year. It never worked 100% after building it because there was a flat line for all readings less than 100ms. There was also a “T?” error showing at the bottom left of the screen […] I finally got it working after cleaning the entire board with IPA to remove extremely small amounts of flux in‑between all components which are packed closely together (which is pretty much the entire board). I don’t use extra flux for my builds but the flux core in the solder leaves very small traces and was apparently causing random short circuits. I had no idea.”