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There are a few different ways to waterproof canvas effectively. This material actually already has some basic water-resistant qualities, and while not waterproof in its natural state, it does lend itself very well to treatments that allow the fibers to repel water instead of allowing it to gradually seep through. In addition, the waterproofing process will help to fend off mildew, which could damage the canvas over time. Building on those water-resisting properties is the quickest and easiest way to make canvas waterproof.
Using a spray-on product that contains a fluoropolymer formula will result in waterproofed canvas in no time. Apply the product evenly and allow the material to dry thoroughly. You can test how waterproof it is by hanging it on an outdoor line or tree limb and tossing a bucket of water on the canvas shell. If the water beads off, then you know the treatment worked. If not, dry off the canvas and repeat the coating process.
When using a spray-on product, make sure it does not contain silicone. The reason for this is that most canvas is at least partially made with acrylic fiber. The silicone sealant will break down the acrylic fibers, leaving the canvas more susceptible to moisture than it was before the treatment.
A petroleum-based sealant is also a good way to waterproof canvas. There are several excellent types of this form of sealant that can be applied to the canvas using a pain brush: simply spread out the canvas and apply the sealant. Allow the product to dry on the canvas, then test it with a splash of water. The canvas should allow the water to run right off.
One firm rule of thumb when using any canvas waterproofing method is to make sure you are in a well-ventilated area during the process. No matter what type of waterproofing agents you are using, there will be some fumes. If at all possible, do it outside. As added incentive, some of the substances that are used for treating the canvas could damage your floors or the finish on wooden furniture.
Keep in mind that there is no such thing as a permanent waterproof canvas coating. Over time, you will need to repeat the treatment again, if you own the canvas item long enough. Under normal conditions of wear and tear, however, you should not have to waterproof items more than once per calendar year.