Can I waterproof my down sleeping bag?

A: Yes, indeed. If one is so inclined, you can in fact enhance the water-repellency of a down sleeping bag with a product called Nikwax TX-Direct Spray-On ($12.50 for ten-ounce bottle). It's easy to use -- after washing a bag (carefully, and in a commercial-type front-loading machine), and before drying it, just spray on the Nikwax. I've used it on several pieces of rainwear, and it works well, forcing water to bead up and run off the fabric before it soaks in. It's perfectly safe for the bag -- the big risk is in damaging the bag by overly aggressive washing. As I said, use a front-loading machine, and a gentle liquid detergent.

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.