You squeeze it out, slather it on, call it a day. Yeah, no. Sunscreen can only do its job — shield your skin from damage — if you do yours. And that involves some education (and, yes, reapplication). These are the nine mistakes leaving you more exposed than you know.

1. You’re not using enough.

“People who apply SPF 30 are usually getting the efficacy of a 10 or 15,” says Steven Q. Wang, the director of dermatologic surgery and dermatology at Memorial Sloan Kettering Cancer Center in Basking Ridge, New Jersey. “Most people apply one milligram — instead of the recommended two milligrams — per square centimeter of skin, so they’re getting about half of the SPF value on the label.” The solution isn’t to buy SPF 100 and keep skimping — you’re better off applying several thin layers of a broad-spectrum sunscreen.

2. You slap it on and leave it at that.

Put on sunscreen at 8 a.m., and you’re not covered for a 10:30 coffee run, much less an alfresco lunch — most sunscreens work for about two hours, says Kristin Nord, a clinical associate professor of dermatology at Stanford Medicine. If you’re inside for the majority of the day, touch up right before you go outside — it’ll give you a little extra protection without messing up your makeup.

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