Your cat has IDIOPATHIC DERMATITIS which some Persians get. Our Raven has the " coffee grinds" under her chin and on the sides of her mouth. I have found that shaving the affected areas and are washed after meals. I even take a flea comb and comb out the " grinds." I add lecithin to her food and that seems to help. Most antibiotics(pills/ointments)have not helped. I am so sorry to hear that your cat is still affected by this!!!!