Did you write the other day that the FA turned some woods black?
Here is some white oak (heart and sap) with FA and ammonia.
Left to right: branch, fumed branch, FA on branch + finish, heart wood w/sap + FA, heart stick cut in half to show how deep the color change is (not very), "raw" heart wood, some bits.
If I apply more than one coat, there is virtually no change after the first coat, like the tannic acid has all reacted with the first coat.
BTW, Apple + FA somewhat like the lower right chip, ie not very pleasing.