[Hammerhead]

I had some Blazer aluminum case .40 ammo go bad after spending 3 days in a wet plastic bag. I got caught in a rainstorm, spilled a box of blazer on the bech in the rain, scooped it into a plastic bag and forgot about it until my next trip to the range. About half of the twenty or so I tried failed to fire (wet primers), those that did fire, fired normally, so there was no evidence water was getting past the case mouths.

Maybe the aluminum cases didn't provide a good seal for the primers.

I seal primers occasionally on ammo I put away for that 'just in case', paranoid driven survival scenario, but I've never sealed case mouths.

But you do (did) see a lot of military ammo with the red lacquer sealant at the case mouth. All that early Russian surplus ammo had it too. Never had a problem with it or heard of it causing any problems.

Premium self defense ammo like Gold Dots are sealed at both ends, but some like Cor Bon are not sealed at either end.

If you want to seal some ammo 'just in case', go for it. The sealant Midway sells is plenty thin and shouldn't bother head spacing at all. Sealing the primers is pretty quick and easy with a small artist's paint brush.