Of course, any good quality down bag already has a water-repellent coating on the shell, so this may be a case of gilding the lily. And keep in mind this process will not make the bag "waterproof." There are just too many needle holes in a down bag, and these always will offer a route for water to get in. Note as well that adding an extra layer of water-repellent coating may adversely affect the breathability of the bag. And that's important -- if a bag can't breathe, then condensation will form inside the insulation, wetting the down.
So I guess my advice is this: Unless you're having some wetness problems, skip the spray-on stuff. Get a slight sleeping bag cover instead, and use it when conditions warrant. Mountain Hardwear makes a nice one called the Conduit SL Bivy. Only $99, and weighs just over a pound.
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