In March 2018 researchers from Denmark published the results of their study to assess whether a double application of sunscreen reduced the amount of skin missed following a single application. 31 individuals wearing swimwear applied a double application of sunscreen and had pictures taken in black light before and after each application. As sunscreens absorb black light, the darkness of the skin increased with increasing amounts of sunscreen applied. Results showed that the individuals had missed an average of 20% of their skin following a single application of sunscreen. However, following a double application the area of skin missed reduced to 9%.
Heerfordt IM et al. Sunscreen use optimized by two consecutive applications. PLoS One. 2018 Mar 28;13(3) [Epub ahead of print]