I've had the same question. I believe that if you seal with natural oils, then that's fine because the oils can actually absorb or allow other moisturizers to get through. Serums contain silicones that can actually coat the hair and block out moisture. I don't know if cones block out moisture 100%, but I know it makes it harder for moisture to get through. From experience, I can't really use products containing mineral oil/petroleum because those ingredients tend to coat my hair and prevent moisture from coming through. Since I've been wearing buns lately, I'd moisturize my ends with water and seal with Qhemet Alma & Olive Heavy Cream or with some type of oil. I also moisturize my hair with more natural products, especially ones that contain Shea Butter. Today, I'm going to start moisturizing my hair with Oyin's Whipped Pudding which has water in it. After using that, I'll continue to seal my ends with QB or an oil.