Primer compound is usually sealed inside the primer cup by a foil or paper disk, and a drop of lacquer. The lacquer not only seals them, but helps keep the anvil in place until they are loaded.
Then when a new primer is seated fully, the primer anvil is crushed further into the cup, the point on the anvil generally pierces the lacquer & foil seal, and if excess force is used, cracks or crushes the primer pellet to some extent.
At that point, the factory double seal (foil & lacquer) have been compromised slightly, but the tight fit in the primer pocket makes up for it.
SO, in theory, a seated primer is more susceptible to moisture then a new one still in the package that hasn't been seated yet.
But, the primer compound was mixed and inserted into the primer cup wet, at about the consistency of tooth-paste. Once the cup, compound, foil or paper seal, lacquer & anvil have been assembled wet, the primers are sent to a drying room where moisture & the lacquer solvent are removed.
So, again in theory, a primer could be soaking wet one day, but as long as it was dried out completely, it would work perfectly the next day.
That's how they were made!
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