Whether or not you actually do it, you know that you should be wearing sunscreen all the time — not just in the summer. It is, without a doubt, the best thing you could do for your skin.However, aside from that hard-and-fast rule, there are a lot of gray areas when it comes to SPF. How often should you reapply? What's the difference between chemical and physical sunscreens? Is the SPF in your makeup really enough?Luckily, we've decided to clear things up once and for all. We tapped two dermatologists — Alysa Herman, MD , and Zein Obagi, MD — to talk about all things SPF. Have a big question? We've probably answered it ahead. Click through — and then slather thoroughly. Your skin will thank you.