I've done this technique for the first Four Roses barrel for Harry out in California. What I do is take the charred flakes after chiseling them from inside the barrel and let them sit in the water based clear coat I use on the outside of the barrel to basically create my own dark stain. You may think it would create an ebony like dark stain but it doesn't, the same reason the charring doesn't cause whiskey, scotch or rum to turn a dark black color. Essentially what I'm doing it recycling and reusing something from inside the barrel to use on the outside of the barrel. In the picture you can see the darker side from the lighter side after one coat, it's almost a dark oak stain that was created from using the barrels own charred characteristics. I chose to do this to this particular barrel because after some sanding, the wood looked to new in color and grain to not reflect what the barrel has been through and what it's created. This is only one coat, I typically use 2.