If you’re using both sunscreen and insect repellent, apply the sunscreen first, then the repellent. If it’s a chemical sunscreen, wait about 20 minutes for it to be absorbed by the skin; you needn’t wait with sunscreens containing physical blocking agents such as titanium dioxide and zinc oxide, which don’t need to be absorbed.
If you use a repellent that contains DEET, keep in mind that it is a solvent and will reduce the effectiveness of sunscreen. A concentration of 30 percent DEET spread on top of a sunscreen with SPF 15 decreases the effectiveness of the sunscreen by about 40 percent, according to one study. So use a generous amount of the sunscreen, and reapply it more often than usual.
Don’t use a combination product containing sunscreen and repellent; sunscreen has to be reapplied often (every two hours) and liberally, which could overexpose users to the chemicals in repellents (which require much less frequent reapplication). And tests by Consumer Reports have found combination products to be less effective at repelling mosquitoes.
Also see How to Prevent Mosquito Bites.