I tried a myriad of spirit based leather dyes, and wood stains, and found most worthless. All seem to fade to some degree. The pick of the litter was Lincoln's shoe dye, which seems to have more dye material in it than Feibing's which had very little. Lincoln's has the interesting ability to be washed out, or blended with water after application, even though it is spirit stain. This makes it easier to blend any overlaps or thin spots. It does sun fade a little, but not at all like Feibing's which I actually had fade while it was in the shop near a window, Lincoln's will wear off of hard maple, with handling. What I use it for more than anything is aging rawhide wraps on war clubs, and rifle stocks. I apply it with a brush, and while it is wet rub it with a wet rag ( not damp, wet) you will be shocked when it is dry, it looks like its been there a hundred years.
I have had great result with Aquafortis, and I do use the Lincoln's light brown dye just before I apply the finish to give it more depth.
The vinegar stains are a new experience for me, and so far they have been very controllable, and predictable, something Aquafortis isn't.
Hungry Horse