This need a pre-wash treatment by gently rub a bar of moistened soap along the length of the dirty mark. Hair shampoo is an effective substitute. Use a clean toothbrush and water to work up a lather.
Rinse and launder the garment when the mark has faded. To help keep collars and cuffs clean between washes use ironing spray or spray starch when pressing the freshly laundered garment.
