Responding to the comment by Graham about solder mask doing a temperary
job.  This was probably true for the origonal use.  However requirements
have far surpased the origonal description and the solder mask does a
permanent job protecting the FR-4 and copper circuitry.  UL lists Solder
masks as permanent polymer coatings.  Conformal coating is used to protect
leads and devices.  Because of the sequence of application one cannot due
the job of the other.

As far as epoxy resins being hydroscopic this is both true and false.
There are many types of epoxy resins and curetives and depending on the
selection the cured epoxy resin can either be hygroscopic or not.  Solder
Masks are tested to very high levels of moisture and do quite well.

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