A couple of things come to mind. I have several issued patents in the area of vacuum insulation panels for which a lot of work has been done on barrier film permeability to water vapor and other gases. It turns out to require multiple layers of specialized films and aluminizing to create good long-term vapor barriers. No single polymer exists that isn't permeated eventually, and even the multilayer films will be if enough years go by. So for long term storage the sealants are basically irrelevant. The relative humidity of the storage conditions are all that matter for that.
A desiccant with a color indicator inside a plastic bag is probably your easiest solution for long term storage. The mil-spec desiccants are mainly smectite clays, like montmorillonite and bentonite. That's what's in the 8 lb. bag of oil absorbing granules sold by AutoZone—100% montmorillonite clay, according to the label. Spread that out on a cookie sheet and pop it in a hot electric oven (400-450°F) for an hour or two (until it stops losing weight) and you've got dry mil-spec desiccant you can put in a big tea ball and leave in a sealed plastic freezer bag with the ammo. Use it to augment the efforts of your color indicator silica gel.
The sealants are really for short term water resistance and resistance to water under pressure, such as the diving scenario Snuffy mentioned. Primers loaded to mil spec are seated pretty firmly, and that can crack the protective foil protecting the priming mix before loading, so the primer can become much less moisture resistant in situ. If you have a case that's been reloaded many times and the sides of the primer pocket are scratched and scored, then you may, again, have a water path to the inside that a sealant can protect. There's some pretty good solvent based silicone sealant available in nail polish brush bottles that is sold for patching dry suits at dive shops.
Many people have found washed rounds shoot fine, but a former Aberdeen Proving Grounds Test Director on another forum described having loads rendered into duds on a rainy day by water drops falling onto the primers of rounds sitting nose down in an ammo box on a bench under a leaky firing point roof. The fall of the drop would increase the fluid pressure at impact locally. But he would also need a cracked internal primer seal for that to be an issue. So, the stars have to line up wrong, but it's just not impossible.
For that reason, if you want to mess with sealant, doing it for field rounds that will be used on an expensive hunting trip makes sense to me. You don't want to risk that an expensive trip could be spoiled by lack of a nickel's worth of sealant. At the same time, I would not be cavalier about applying sealant either. If water can leak in, so can sealant solvent, and it may attack protective lacquer the primer maker applies. For that reason, I would seal primed cases that were not yet loaded. I would let the sealant dry, then put the cases into a sealed bag with some of the clay desiccant (it gets a number of solvents pretty well, too). Let it sit for a couple of weeks, then load and test fire enough cases over a chronograph to have confidence the primers are not performing any differently than they did before sealing.
Gunsite Orange Hat Family Member
CMP Certified GSM Master Instructor
NRA Certified Rifle Instructor
NRA Patron Member