We would still avoid applying any hair product, sunscreen or insect repellant directly to a helmet as we did in these tests. And if you use sunscreen or insect repellant on your head before riding you would always want to wash it off the helmet with dishwashing detergent and water afterward, or just take it in the shower with you. But the lab tests demonstrated that minor surface effects from incidental contact are not likely to significantly degrade a helmet's impact performance.
Full report at Helmets.org
Andrew