Ok, been pulling my hair out over this. I just worked on a holster and I think I have figured out this problem. This is a dye penetration issue, not a sealing issue. Remember here folks, we're working with dye, not paint. Dyes penetrate the materials. Paints sit on the surface.
The short & simple answer is dye must be applied to leather that has not been pressed/compressed.
For example, like this:
I dye the leather under all mouth reinforcements and belt tunnels before they are glued and stitched on. I do this to ensure that glue doesn't interfere and resist dye, leaving a glaring undye-able area in these tight sections. Once these initial pieces are stitched on, the construction continues. Glue the holster, edge, stitch and wet mold (which includes pressing). That bring us to a dryed, molded holster ready for dye.
SORRY FOR THE NOT-THE-CLEAREST PICS
Now I dye the holster, then buff. Here is the result:
You can see in the yellow circle the difference in how much the dye penetrated the leather. The darker area was dyed prior to wet mold (as in the belt tunnel picture up there). The lighter area (and entire holster for that matter) was dyed and buffed, several times, after wet mold. The only explainable difference is the structure of the leather. Before wet mold the leather fibers are porous and open. After a wet mold that includes pressing the leather, the fibers are compacted and tight. The dye just cannot penetrate the compacted fibers (even diluted). I tried several coats of dye and buffs and was left with that result above.
Well....heck...I need to save this holster somehow. The solution? Saturate the inside of the holster using a dauber. Saturate the heck out of it until you can see the dye puddling in there (if you turn it over and look on the outside you should see the dye seeping through the grain). Now reapply dye to the outside of the holster. Wait a minute or two so that the dye has time to work in there and meet in the middle. Now force dry with a hair dryer (this will minimize wet time which slightly softens the interior molding detail). It won't take long since the dye is alcohol. Buff the piece you just dyed and inspect the leather. You should have pretty much have it covered now. If you're satisfied with the results, do the other side the same way. Once its dry, buff well.
Here is the result:
As you can see the dye job has evened out quite a bit. It's much better than in that first pic and does save the holster. Now the holster can be finished up knowing the dye is "in there" instead of just sitting on the surface.
With all the above information said this brings into question the order of construction on holsters, specifically black ones that show such a color contrast.
Some makers dye leather before doing anything else. This is the best way to ensure solid, thorough dye coverage that will not rub off. However, I don't think this is not without consequence. It's highly likely that you'll get dye residue all over everything involved in your construction process. Things like your hands, your sewing table/platform, and your press pads to name a few. Also if you bone your holsters, the dyed surface grabs the boning tool, making it difficult to slide the tool over the leather for nice lines. I have no idea if the dye residue concerns and smooth boning issues can be worked around. I would like to hear further input on these two things from those who have found solutions.
If you wish to dye holsters after they are wet molded it seems you can still do that using the method of saturating the interior of the holster, then dying the outside. The results should be acceptable.
If you do not press your holsters during wet mold you might not experience such a drastic dye hurdle.
Ok, those are my thoughts and observations. Hope it helps.