2 things. i know they offer slip-on type covers for packs to keep em dry. if your looking for something just to apply to the bag itself, i think Scoth-Guard (starch) makes water-proofing stuff too....check it out. i used to spray the stuff on my boots.
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"It is a far, far better thing that I do, than I have ever done; it is a far, far better rest that I go to than I have ever known" - A Tale of Two Cities - Charles Dickens