However, if the canvas bags is not waterproofed, water will seep through it, resulting in mildew build up. Cotton canvas has been the popular waterproof canvas fabric for a long time. The cotton fibers in the cotton canvas swell, when they get wet, thereby sealing the weave. However, cotton canvas has a special affinity to mildew, thus it has been replaced by either woven acrylic or vinyl-coated polyester. The vinyl coated fabric unlike the traditional cotton canvas does not breathe, and needs to be ventilated. Condensation causes the fabric to become wet, eventually leading to mildew. On the other hand, the acrylic canvas is a tight weave fabric, which breathes; stands up to ultraviolet radiation, snubs mildew and resists staining. The only negative factor is its high cost.
To waterproof a canvas bags, one should begin by cleaning the bag thoroughly inside out. Special attention is to be given to hidden seams in the bag. Small amount of rubber sealant, like the ones used to fix holes in rubber shafts is to be applied along the entire seam. Spray a silicone-based waterproofer on the entire surface of the bag, giving special attention to the bottom and seams of the bag. As far as the zipper are concerned, no zipper is 100% waterproof. If water seeps through the zippers of the canvas backpack, one can use acid-free packing tape to deal with the leak.
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