Ok I got this thing and it works pretty good after you have it all set up you just put it over the stain turn it around. The Acid flows right on top of the stain and you can see the yellow color when the Acid starts reacting.

Well the problem is I can't get the stains of the plaster. The stains are brown and some are dark nearly black. I tried Vitamin C, Chlorine and Acid with not success. All with direct applying the stuff on to the stain.

Does anybody has an idea what else I can try?

Guido