Are you using an oil or some sort when you are rubbing it back? I used a scotchbrite pad and some Murphy's oil soap to rub back the black of the tannic acid on my last build and it came off very easily where I wanted it to. The tannic acid definitely turned VERY black, but I'm surprised it isn't rubbing away easier for you. There were areas on mine where I hade to rub extremely lightly to keep any of the dark look at all.
Also, I don't know that it would matter, but did you apply the AF or tannic acid first?
I believe my order of operations was:
- Apply AF coat and blush
- Apply AF coat and blush
- Apply tanic acid and blush
- Almost immediately began rubbing back the tannic acid black with oil/scotchbrite