One of the harder materials to waterproof is down. Even if you have the best waterproofing for jackets, down is usually found inside shell fabric, making it harder or near impossible to reach with waterproofing spray. If you specifically want to waterproof the down, then simply use a down-specific wash-in product.
What’s special about down-specific wash-in is that they are specially formulated to be gentler than the average detergent. They can revitalize the down’s insulation and water-resistant properties. This is the best solution for waterproofing a jacket or any other outerwear with down.