It doesn’t matter how high the SPF is or how “long lasting” the label claims it will be — sunscreen doesn’t stay with you all day. Ironically, what degrades and neutralizes sunscreen is sunlight. “When you’re in the sun, the sunscreen will break down, and depending on your activity, it may also get washed away with water or sweat,” says Henry W. Lim, a dermatologist and photobiologist for the Skin Cancer Foundation.

The experts recommend, as a general rule, fully reapplying over your face and body every two to four hours when you’re in the sun. Sure, even a little is better than nothing, but you’re not going to get the best protection if you don’t use enough sunscreen. “Studies have shown that people use about half the amounts at which the products are tested,” says Lim. “So the label may say SPF 30, but you’re getting closer to SPF 15 or 12.”

For complete coverage, the experts recommend a shot glass–size amount (one ounce) for the entire body and a pea-size amount for the face.

According to the saying, a picture is worth a thousand words. But if you are brave enough to have your dermatologist take a UV photo, that’s a picture that will shock you speechless. These special images are almost like X-rays of the skin, revealing all the sun damage that’s not yet visible on the surface. Dermatologists agree that the main purpose for these is educational. “Even on a 15-year-old, it would show a lot of blotchiness, so it’s a way of educating people who can’t appreciate the damage they’re doing,” says New York dermatologist Diane Berson. “It’s also really scary-looking,” and this helps doctors show patients the importance of vigilant sun protection.