You cannot mix our Tulip Dew Sunscreen Serum with foundation as it is mineral, meaning it creates a physical barrier for protection. If you mix the sunscreen with foundation or another skincare product before applying, it disrupts the level of protection. With all sunscreens you should reapply sunscreen at least every two hours. The only time you will need to apply sooner is if you go swimming or if you exercise and sweat. Our sunscreen is water resistant up to 40 minutes but once you dry off you should reapply immediately to ensure you are getting full protection.