The procedure is roughly the same for both Down and Synthetic, however down requires special precautions:
- Never dryclean.
- Wash by hand in a bathtub, or use a front-loading washing machine on gentle cycle. Down bags have thin baffles inside that keep the feathers partitioned. Agitators will put enough stress on the bag you risk tearing those baffles resulting in the dreaded clump.
- Use non-detergent, no-perfume, non-bleach soap such as Nikwax designed for washing down. (There are common non-detergent laundry soaps as well). Down is a natural organic material, and hard detergents / bleaches can damage it.
- When drying, use low heat (or no heat) on gentle cycle, and toss in a couple of tennis balls. The balls add some extra non-destructive ooomph to the process that helps break up clumps of down. Check regularly and pull apart any down clumps by hand you encounter.
Sadly, despite being as careful as possible, washed down almost never seems to be as good as new. So do it only when necessary - if ever. Smelly bags can often improve just by laying out in the sun for a few hours. UV is brutal on funk-bugs.
PROTIP: Using a sleeping bag liner (silk or other) that will intercept your body's dirt and oils (the down-killers) will extend the life or your bag.