Shellac or nail polish will work. For primers, I dip the tip of a fine haired artist brush in the stuff, after thinning about 2:1, and put a small drop at the juncture of the primer and case. If it is thin enough, it runs around the primer ring under its own flow tension and you can see it if you use colored nail polish. I use different colors of nail polish to differentiate powder charges.
The same can be done at the cannelure on the bullet after you crimp it in place. I only do this to ammunition I expect to use in wet conditions, but it has handled total immersion of my shells for periods of up to six hours (worst case, stuck in the bottom of a jon boat that was on the bottom of a river) at six feet. Ammo fired just fine after I dried it off and did not change point of impact according to my scope zero.
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