My RB tuxie, Peeka, had really bad dandruff like that for a while. My vet gave me a spray to apply directly to her coat; the spray was really oily and, unfortunately, smelly but it worked. This was many years ago and that vet has long since retired so I'm sorry that I can't tell you the product name but if you ask your vet about a similar product, s/he can probably direct you. Good luck.