Can you waterproof a "down" sleeping bag?

I just bought a Kelty 20 degree down sleeping bag. How can I make the fabric waterproof? Can I spray it, or wash it with waterproofer? Will it hold up?
What's the "downside"?
Insulation 650-fill-power goose down
Inner Shell P300T polyester microfiber pongee
Outer Shell PR349T polyester diamond cross-dye ripstop

I assume that you are talking about waterproofing the outside of the bag to keep rain water from getting to the down. The problem is that waterproofing a bag will also make it vapor (sweat) proof and you will get condensation on the inside of the waterproof fabric. This condensation will wet the down and the down will loose its insulation properties. You could also waterproof both the inner and outer fabric. The down would stay dry but you wouldn't.

Thanks, Pika. When I read about these waterproof down bags (outside waterproofness) They claim to be "breathable" waterproof fabric.
Can I treat my bag to be breathable?

I would get a waterproof bivy/sleeping bag cover instead of treating the bag. That way when you use it under normal conditions/nice weather, the bag breathes to let your perspiration out rather than collecting it, and in wet weather you can tuck the bag into the waterproof cover and dry it out fully later.

People get paranoid about down getting wet. If you're somewhere humid or rain breaks out and you haven't somehow protected the contents of your pack, I can understand worry. In a campout in the Sierra (for example) where weather is unpredictable but not monsoon-like, keeping the bag in a dry sack and using a pack liner or cover then only unpacking the bag inside the dry shelter out of the rain shouldn't be difficult.

Yes.

1) Remove existing outside fabric layer

2) sew on a new waterproof and breathable outer fabric layer.

The sewing part might get a little tricky, as you have to sew to the interior baffles and not let all of the down blow away while you are working on it.

I'll second the bivy concept. But in reality, why do you want to waterproof it? If you use a propper shelter you should not need a waterproof bag. If you use a mini tarp then maybe the foot box, but even then, making your mini tarp larger would weigh less than the W/B fabric foot bivy. A little mist is not going to hurt your bag unless its realy cold. And if its realy cold, a better shelter is needed.
I'm not up to date on what materials are used, but they are not normal gortex, they breath better and somehow are just as waterproof (?).
Scott