I usually apply Aqua Fortis with a paint brush. The stuff I use was made by a member of our guild. Sadly, he's now passed. I cut it with water, which doesn't seem to change the effect, but makes what I have go farther.
I like to blush it with a heat gun... then let it sit a day or two. Sometimes that gray-green color comes back after sitting overnight. Then I hit it again with the heat gun before applying finish.