Originally Posted by CliftonGK1

Wash-in waterproofing (like Nikwax, by far the best brand) will re-proof DWR material that's lost it's oomph and give some water resistance to tight weave fabrics like cordura and ballistic nylon packs. If you're talking about trusting it to fully waterproof a pack that you'll use for courier work or commuting, don't. It just won't hold up to the abuse, especially when you throw dirt and road grime into the equation.