"Hair serum is usually seen as a finishing product, but it works well when applied before you wash your hair," Bailey says. He recommends putting a few drops in your hands before rubbing them together and simply running it through the lengths of your hair. "Some of the product will come out in the wash, but a lot of it doesn’t, and what isn't washed away stays within the cracks and smooths the hair surface," Bailey says. "This adds a level of protection to the hair when you’re washing. I liken the hair strand to a scratched table: When you’re repairing it, you pile on the polish, but you remove 99% of it, and what’s left plugs the gaps and leaves you with a nice polished surface."