Sgraffito can be very effective for painting hair, or rather 'drawing back' into the paint to create strands of hair. Depending on what size object you use, you can get marks of varying width, from very thin to represent individual hairs to thick to represent bunches or highlights.
In the example shown here, the colors had gone rather muddy as a result of overmixing them on the paintings. Being in acrylics rather than oils, scraping back right down to the canvas wasn't an option as the lower layers of paint had dried already. But rather than paint over it, sgraffito was used to create the impression of hair, facial features, and the shirt.
The resulting painting isn't a masterpiece, but it does have a great feeling of texture. Imagine how it would look if the colour of the hair had been more intense.