I've used a product called Dykem for many years now and have had excellent results.
Dykem is a steel marking dye used to highlight scribe markings. It's very fast drying and comes in red and dark blue. Any good industrial supply store will have it, like Graingers or MSC Industrial (both have web sites) and Grangers has branch stores in most major cites.
The nice thing about Dykem is that it's very thin only slightly thicker than water. So what that does for you is that at any place where moisture or oil may find it's way in the Dykem will also work it's way in through capillary action and form a seal.
I have used two methods to apply Dykem the first if you want to coat the entire primer and ring the case mouth get the smallest artist brush you can find. Or (my favorite, although somewhat messy ) I fill a small syringe with a good size needle on it and use the needle point as you would a fountain pen.
The good thing about Dykem is that you apply it to finished ammo, so there's no worry about contamination of powder or primers.
Also as it goes on so very thin and adheres so well there are no chunks to fly off and float around in your gun like with nail polish.
Cleaning it off the brass is easy, tumbling gets most of it and and a quick bath in laquer thiner, acetone, gasoline or alcohol will remove the last of it, if you just have to have spotless brass!
If everyone thought like me, I'd be a damn fool to think any different!