To ensure that you have full coverage of a sunscreen, you should apply an amount equal to a shot glass. Studies show that most people apply only half to a quarter of that amount. As a result, the percentage of SPF they wear is lower than the actual one indicated on the package.
During a long day at the beach, the Skin Cancer Foundation informs us that we should use about a quarter to half of the product. Sunscreens should be applied 30 minutes before sun exposure so that the ingredients have time to activate on the skin. The first application is just as important as the ones that follow, so repeat the process every two hours. Repeat immediately after swimming or if you are sweating.