After currying, and a good brushing, sometimes there's still that dust trapped in their thick coats, that a curry just stirs around, and then gets pushed back into the coat, when you brush it.
I've had luck with Shapleys waterless shampoo.
I'll curry him really good again, and the while the coats all scruffed up, I'll take a rag, spray the Shapleys on there, and start getting the dust up that way. It does a great job of picking up and clearing off the dirt.
You could probably achieve the same, with just a lightly damp rag as well. Main thing is, picking up that stuff the brushes aren't picking up.
"You're just as sane as I am."~Luna Lovegood.