/ waterproofing a rucksack
You might be able to make water bead off the surface to some extent but it will still leak.
Buy a selection of roll top waterproof stuff sacs. You can organise your kit and be safe in the knowledge it will be dry.
Alpkit do some good waterproof bags and also 20L, 30L etc 'canoe bags with straps' called Gourdons
I think ortilab (sp?) make some proper rusksacks, that are waterproof, as apposed to a dry bag with straps if that makes anysense? How ever id probably still use dry bags just to help organise a bit!
With your current bag, you could try the silicone spray, breathabillity is less an issue as with clothing, you could also shove a load of wax over the zips but it will still let in somewhere! Nikwax is a water based product, it maintains breathabillity but wont have a good a barrier as a silicone product!
Thin, flexible waterproof dry bags are definitely the answer for bits that need to stay dry. Or a large one that fills most of sac.
Apart from the Ortleib truely waterproof bags, I've found the Macpac sacs to be most water resistant - think they're a cotton/nylon mix, so cotton swells when damp and fills in the holes. Ish.
+1 to the replies who recommend roll-top dry bags. I use Exped dry bags. Available in a variety of sizes so you can organise your kit as to how you want. This has the advantage that if you need to open your rucksack when it's lashing it down, you only have to open the bag that you actually need.
Ortleib make a big black dry bag that comes in different sizes that you can put everything in. When that gets holes, use seam sealant to repair. Or you can use seam sealant on your rucsac (fill it with water, find the holes, dry it and fix the holes/ tape the seams,) which will waterproof it, but only until you put the next hole in it!
Line the sac with a big, tough poly bag - cheaper than roll tops, probably lighter, easier access to contents of the sac and more useful in an emergency. I sometimes put my sleeping bag in another, smaller poly bag.
After exhaustive testing and spending about 200 days a year out in the pi##ing rain that is loosely described as British summertime, I can say Exped drybags are the way forward for me.
Get a few colours for different bits of kit as it makes it easier to pick the right bag at a glance.
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