Can I make my down jacket waterproof by applying a waterproofing spray on it?
No, to make an item waterproof you need a waterproof membrane (or a chemical PU coating) such as eVent, Gore-Tex or Pertex Shield and you need the stitching to be taped on the inside.
Most of our down jackets don’t come with any of these features for good reason, they are generally designed for cold, dry weather (primarily alpine environments) where snow is the main factor and water-resistance is enough to handle it. Making these items waterproof would significantly increase their weight and their compressed size as well as reducing the mobility of the wearer. All of these are factors aline climbers are keen to reduce as much as possible.
Any products that claim to “make your jacket waterproof” are simply re-applying the DWR coating to the down and the fabric which will increase its water-resistance by reducing the amount of water that is absorbed, but it will not make an item waterproof.