Waterproofing a backpack

Hei Jason, I don’t think its necessary to spray it. Actually I have never heard anyone spray his backpack with some DWR spray. Most likely your pack is waterproof to some extent, and if you want to make it really water resistant then a pack cover could be an option. Or you pack the gear which needs to stay dry at all costs into water resistant stuff sacks. MLD make for both suggested options some very lightweight versions, so go and have a look =)

Or use a trash compacter bag or polyethylene bag as a liner. Cheap and easy to replace.

Your pack has coated cloth and will repel some water, but the waterproof issues have more to do with the seams and openings. I second the notion of using a liner. BPL sells some light ones (http://www.backpackinglight.com/cgi-bin/backpackinglight/bpl_pack_liners.html). Some use trash compactor bags as they are really tough. I use waterproof stuff sacks for critical gear like insulation and sleeping bag. IMHO, rain covers still allow rain to get down the back so a pack liner still trumps the cover and keeps your stuff dry. I use a poncho for rain gear and that keeps my pack dry at the same time.

There are packs made specifically to be waterproof– for canyoneering, kayaking, etc. They are usually much heavier, not as comfortable, and lack features like pockets, etc.