Have you started the hair from the base of the forehead? If yes then start painting it from the top side, drag the paint forward and then dissolve it to the darker are near the forehead. We tend to paint from the hair growth area but we need to start from the lightest areas then lose sight of it in the darkness. Study Zorn's paintings, you'll learn the technique.
@Bancroft414 most of the time the plein air paintings need reworking because even if you captured the fleeting moment, after returning you will realize that there are mistakes or pictorially it might not be a successful one. So you make changes but the basic information is already there. You just start bumping paint by putting more light or shadow to turn it into a good one. Only work on accentuating things, light against shadow and shadow against light; correcting shapes. That's how it works.
For en plein air I believe that it is better that the artists go out and find out how they want to do. When you are out there the nature starts to paint. It is very different from studio practices. But you need to have an idea about how you can execute.
Thanks very much @Summer I use sgrafitto in almost every painting. But you don't get tot see them because they are camouflaged. I have been using this technique since 1996. It is very effective. But most of the time i use my pinky nail or brush end to do this.