To ensure that you get the maximum protection from your sunscreen product, apply sunscreen prior to going out in the sun. You generally need approximately 1 ounce (the equivalent of a shot glass) applied to the overexposed areas. Then follow the recommended reapplication guidelines mentioned previously above. Another important fact to know is that no sunscreen is truly waterproof. Some products are more resistant to coming off when exposed to water or sweat, but the best defense to ensure that your sunscreen does not come off your skin when exposed to the sun is frequent reapplication, particularly after swimming, to avoid any risk of unintended exposure.