A: A chemical sunscreen (one that contains oxybenzone or avobenzone) should go on clean, bare skin first, says Laurie Polis, MD, director of Soho Skin and Laser Dermatology in New York City. You need to give it time to be absorbed, because in order to be effective, it must interact with skin cells. (A sunscreen with a physical block like titanium dioxide or zinc oxide can be applied last, after a serum or moisturizer.) Apply a serum next and, finally, moisturizer if you need it. Polis also recommends serums or sunscreens with moisturizers built in because they save time.
Bottom line: Apply chemical sunscreens or serums that need to be absorbed first, barriers (like moisturizers, makeup, and physical block sunscreens) last.