Contrary to popular belief, though, down bags actually hold up pretty well in the wet. They don't if you're just sleeping in the bag in the open. But if you're sleeping in a tent or similar rain shelter, it takes a long time for a down bag to soak up enough ambient moisture from damp air or occasional splashes of rain to really lose insulation. Yes, it CAN happenshock, down bags can indeed get wetbut if you're careful, your bag should work well for days.
Now, can you do anything to ensure the bag operates at peak efficiency for as long as possible? Well, sure. The bag almost certainly has a shell that was treated for moisture repellency, but you can still supplement that. Nikwax Downproof ($11 for a bottle here in the U.S.) is an interesting choice. It's basically a water-soluble wax that adds a light, water-resistant coating to the shell and down fill of a sleeping bag. That will help the shell to shed moisture, and prevents the down from absorbing moisture more quickly than it normally would. But, as a wash-in chemical, it also "adds" something to the down. I suppose it may slightly reduce the down's loft, too, although I've not heard any complaints about this. So that's one choice.
The other is a spray-on DWR (durable water-repellent finish) similar to what was put on the bag in the first place. Nikwax TX-Direct ($12.50 for ten ounces) fits the bill here, and although it's not specifically recommended for down bags, I see no reason not to use it. It'll help bead up water that falls on the bag when the tent door is open or when you're packing and unpacking it.
I wouldn't use a bag cover, as that may really cause problems by trapping moisture from your body.
My own feeling is that the bag will work just fine, as is. But, for extra peace of mind, a shot of TX-Direct will do no harm and might even do some good. I'm a little less keen on the wash-in stuff as that fundamentally changes the nature of the down.
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