I have used several times on multi story condo hallways with laundry rooms on every floor. Most of them at one time or another leak into the hall and leave a ring. The 30 vol will help. You can also mix 6-8oz of releasit with 6-8oz of 30 vol also. I have used both citric acid and 30 vol combos successfully.
Reducer, Is that what you would try first Willy? Potato whitener huh? That’s interesting. I have never known how to by reducer locally. I guess I better decide whether to start with a reducer or oxidizer as they are opposite ends of the spectrum.