Originally Posted by LWaB
Cotton swells when damp, so the holes tend to close up.
Waterproof is a big ask of natural fibres. Stick with Ortlieb if you are going to get hung up on waterproof.
Carradice are beautiful, high quality, and very classy, but not entirely waterproof. It's just canvas, like an old tent. They leaked.
Bockman and MichealW are right. Here's how I sealed a canvas rucksack to reasonable watertightness:
Heat it in the oven to very warm. Melt SnoSeal in the microwave and use a paintbrush to paint the hot bag on the outside, paying special attention to all seams and stitching. It will soak thru so no need to do the inside. If you do, you may
find some waxy residue on your belongings in hot weather. SnoSeal does not harm canvas or stitchings. Test it for color changes first. It darkened the old bag, but not to an unpleasant degree. You can 're-activate' it by re-heating and applying additional light treatments later, probably only to the top and seams.
outdoor clothing and Barbour
are very good products and are referred to as 'oilskins'. SnoSeal is available at Sears shoe dept or just about any Work-n-More store for about $5 a jar. You get about 3X as much as the Carradice wax.