My two bits .... Along the same lines to what Joyce is saying, hermetic is NOT plastic, gaskets, rubber seals, etc. By comparison, an all-glass envelope similar to an old-fashioned Edison light bulb is closer to hermetic - depending upon the quality of the glass to metal seals (materials used).
Many people use parylene (NOT hermetic). I also believe there is a newer coating which essentially uses alumina to create a hydrophobic coating.
Sorry, I'm a purist with too many years with truly hermetic devices - glass to metal seals and metal to metal welds. NOTHING goes in and NOTHING goes out, if you've done you job correctly. Everything else is just a matter of time. :-)
Steve
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