I guess what I meant is that on its own nylon absorbs water at a molecular level. Cotton absorbs far more obviously. but wet nylon still gets heavy.
As for my comment that waterproofing the fabric isn't enough, Its always been my experience that unless a pack has been engineered to be waterproof, there will always be a way for water to get in. Some seam that is hard to seal, a zipper that catches water, that sort of thing. Its all a matter of degrees, Very water resistant might be enough, but my definition of waterproof involves a full dunking. Just the kind of stuff I do, so its how I think about it. To each their own though. My definition of heavy rain changed a lot when I moved to the tropics!
Given materials and construction, I can agree to that. Even the best "brand new" SilNylon tents will have some degree of seepage...
The pack I have is an old Woodland Camo, IIFS Field Pack 8465-01-286-5356 pack with internal frame:
I'm missing the Combat Patrol Pack NSN 8465-01-287-8128 that attaches to the back though:
The Pack also appears under the alternate name of "CPF-90 Field Pack."
...and you are absolutely correct in saying water will eventually get in - but my experience [with this pack] has been that happens most when the bag isn't correctly closed up.
When I waterproofed that bag, I used my previously posted concoction. There were some spots where the paste wouldn't absorb and had to be wiped off...but overall, it worked very well.
There are no zippers but there are draw strings and straps/buckles. The only time I got water in my pack [after waterproofing] was during a torrential downpour that was actually blowing sideways. Think "monsoon" intensity winds and rains and you get what I mean...lol.
I had too much crap stuffed in the bag and the "lids" didn't quite cover the pockets properly. That's my fault though, not the bag's and certainly not the waterproofing agent's. I have to learn to carry less crap into the field LOL.
That bag was produced with a heavy nylon fabric with an application of [what I think is] latex on the inside of the panels. It's incredibly abrasion, as well as water resistant without alteration...but mine is suffering from the [Latex?] lining/backing cracking and peeling failure.....probably due to age and use. I got this back in the early 90s