We were skeptical at first — if the powder just sits on top of your skin instead of absorbing into it, how does it not just wipe off the minute you walk outside or touch your face? "A well-made, high-integrity mineral powder sunscreen will stay in place because of the particle size," says Dr. Chiu. She adds that the skin's natural oils also help it adhere to the skin. That said, she recommends that you should still reapply every two to three hours, or after swimming or sweating. She also notes that because the powder sits on top of your skin, it's important to make sure your other skin care products are completely dry and absorbed into the skin before you apply your powder. Otherwise, your protection may not be as effective.