Of greater concern than a sealant is the environment in which you will store the ammo.
Store it in a shed out back and you're inviting trouble. Sure, it may be protected against moisture by using a sealant, but not against wide ranges of temperature!
Temperature, especially high heat, is the real enemy of smokeless powder. Granted, high humidity might creep in around a bullet or primer over time but if the primer or bullet are a good fit it's not so likely.
Keep ammo in your gararge, vehicle or shed and summer heat will adversely affect the smokeless powder, perhaps the primer as well.
I've always kept my ammo in my spare room, in the house, where temperatures are pretty much constant throughout the year.
I wouldn't worry about sealing the primer or bullet. Worry more about exposing it to a cycle of high heat and bitter cold.
Interestingly, black powder is more forgiving when it comes to such extremes. But the primer may not be.
Store all ammo away from such extremes and it will be good for decades. Store it in such extremes and it may go sour in months.