It's been years since I played with it, and I used two types. The first was back in the 60's and it was a black tar or asphalt type stuff supposedly used for military ammo at one time. Kind of a PITA to apply. Next was a few years later with some clear stuff that reminded me a little of nail polish, and could be applied after the primer was seated rather than before. Better to apply.
But I ran some tests on pistol ammo with both. Went so far as using them on both bullets and primers. I dumped some of each along with some factory loads and some handloads without into water and leaving it there 1 hr, 4hrs, 8 hrs, 24 hrs and 48 hrs. Everything went bang whether in the the water an hour or two days. I quit the experiment at that point and quit going to the trouble of using either.
Other's mileage will most certainly vary, and I bet it depends a lot on primer fit. Might be interesting to try dumping the ammo in a tub of WD40 and trying that.
I only use it on my waterfowl shotshells. Actually just used a clear fingernail polish & apply after primer seated. Also used nail polish to seal the crimp end. Never had a misfire problem even after dropping an entire box in a water puddle. Don't know how long they were in the water, but they all fired.
I have never used sealant either and I have been reloading for many years. But....if you had just laid in a bunch of money for a hunt, would the time spent prepping your ammo for that hunt be worth it? If my brothers could convince me to go Southeast for black bear or deer I would seal my ammo. Just in case.
Well… basically we are talking about an interference fit between the primer and the case and the bullet and the case. That interference is usually a few thousandths of an inch or more between similar materials that are generally quite smooth.
Now lets think about the brass freeze plug in your trucks engine…. Same thing, interference fit (no goop or sealant needed) but still able to seal well enough to prevent leakage under pressure and at high temperature for extended lengths of time.
In my opinion primer and bullet sealant is a waste of time, effort and money. Water molecules are simply too large to pass thru the microscopic grain structure were the two materials meet during the interference fit…..