Sunscreen application is a mainstay of sun protection behavior worldwide. 1 Sunscreen users, however, misapply sunscreen in a number of ways, including failing to apply it 15 min before sun exposure, failing to cover all exposed areas, failing to apply it thickly enough, and failing to reapply it as directed. 2 Sunscreen reapplication is important to replace product that may have been rubbed off by clothing, 3 physical activity, or contact with sand or other objects. 4 Furthermore, application of a second layer of sunscreen has been recommended as a means to compensate for initial underapplication. 5 Failure to apply a thick enough layer is problematic because it negatively affects sunscreen product performance. During manufacture and testing, the Sun Protection Factor (SPF) of a sunscreen is determined by testing the product according to standards set by the Food and Drug Administration (FDA), which specify a testing thickness of 2 mg per cm squared. 6 Internationally, the ISO 24444-2019 standard is similarly used to evaluate sunscreen SPF. 7 Research