D.B. Cooper asked:
Is any of this true, or is it internet fantasyland?
It is true that there are commercially available sealants for ammunition, but you don't put them in the primer pocket before seating the primers, you use the sealant to paint around the base of the primer AFTER it is seated. As to sealing the bullet, you would "paint" the sealant around the base of the bullet and then seat it.
But, what when you do this, what you are concerned about is
NOT waterproofing the cartridge. As
ColtPythonElite said in post #2, ammunition is pretty well waterproof as it is. And, tiny amounts of water vapor does not "deactivate" primers of powder. The reason the military seals its ammunition is to prevent incursion of petroleum-based chemicals and solvents that
CAN affect powder and primers. Unless you intend to bathe your ammunition in huge amounts of WD-40, you probably don't need to seal it.
I
DO "seal" my primers by "painting" fingernail polish across the case head and then wiping off the excess with a paper towel. But this is not done to keep out moisture or solvents. It is done 1) for cosmetic reasons since I use black fingernail polish and think the black ring around the primer looks nice, and 2) to identify unusual rounds/loads. Not too long ago, I bought some primed brass. I mistakenly read the advertisement to say that the brass was new and had been pulled down. As it turned out, it was actually pull-downs of "factory" reloads and was quite disappointing. I "sealed" the primers in this brass with red fingernail polish so that I can readily identify it.