It's hard to escape the sun beating down on your neck and arms during a long stretch of trail with little tree coverage. Protect yourself from the sun without needing long layers.
Banana Boat Sport Performance sunscreen is a trusted brand. It doesn't run, and it is sweat and water resistant. It applies smooth and clear, and has a mild odor. Best of all, it comes in a 1 fluid ounce tube which is the perfect size for a week or more on trail depending on how much skin is exposed and how often it is reapplied.