You just have to keep at it until it's totally off. Sometimes that means soaking it and picking it off with your fingernails...I know, yuck, right? But it's essential to get that gone. After you have removed it, coat the area with just a little bit of olive or coconut oil. That keeps poop from re-sticking to those little downy feathers back there. Some folks even use a manicure scissors and trim the down away from there, but I don't like to do that because I think it presents a nice pink, bare target for the others to peck at. You may have to repeat it a time or two. I had two who got pasty butt almost right after they arrived and I had to do them twice, but like so many others I"m convinced that MHP, the even, constant heat, and not eating 24/7 just because they are awake and the food is there really cuts down on pasty butt! How many chicks under actual broody hens have pasty butt? Nobody I've asked has ever seen it broody raised chicks, and since we are trying to duplicate as closely as possible those conditions it stands to reason our incidences of chicks with that annoying problem would be less too. Edited to add: Nice job on the heating pad setup! Your dues into the Broody Brigade are considered paid with the photos!