When a layer of dandruff appears on the scalp after washing hair, this condition may be a manifestation of seborrheic dermatitis. Generally, after seborrheic dermatitis appears on the scalp, it may cause inflammatory reaction of the scalp, and the inflammatory reaction stimulates the epidermis and the stratum corneum above the epidermis, which may cause the stratum corneum to proliferate. When the hyperplasia reaches a certain level, scaling may appear, especially after washing the hair, the scaling may be more obvious, and you may see a layer of dandruff on top of the scalp at this time. So for the symptoms of seborrheic dermatitis, you need to use anti-inflammatory topical medication, such as compound ketoconazole shampoo, and wash your hair once every 3-4 days.