Seconding Mina on this. It makes a huge difference, especially if you wear eye makeup or have oily skin. You just have to make sure you get one specifically balanced for extensions. I’ve been using the one from Stacy Lash for a while now—it comes with a soft little brush that really helps get between the natural lashes without snagging the extensions. Whatever brand you pick, just make sure to rinse thoroughly with cool water afterward!