How can I remove a sunscreen stain from red and navy cotton t-shirts that remained after washing?

During summer, this is a very common stain! I suspect the oily part of the sunblock formula is what remains on your shirts, and you should be able to get this out as long as the shirts have been kept out of the dryer. Try applying a little liquid dishwashing detergent directly to the stains, and gently rub it in. Wait 5 – 10 minutes (but don’t let it dry on the fabric!) and then wash the shirts in hot water and a good detergent. Air dry the shirts and check for success—this is important to not heat-set stubborn oily stains that would come out all the way with an additional treatment. Also, air drying will help preserve the color of your dark t-shirts.