How to Waterproof Canvas
Untreated canvas in it natural state is water resistant, but is not water proof. If you want to prevent the seepage of water through canvas, you need to apply a waterproofing agent to the surface to repel water, to help prevent mildew and to protect the fibers of the material.
When choosing a waterproofing agent for your canvas be sure to look for one that states on the can that it can be used on canvas.
Avoid silicon based sprays which not only won’t work well on canvas, but can actually break down the acrylic fibers and leave the canvas worse off than when you started. Instead, look for a fluoropolymer-based waterproofing spray.
These sprays can usually be found at camping stores or in the outdoor/recreation section of most department stores. Depending upon the size of the item you are waterproofing you may need several cans to cover it all and ensure that it is well protected from the elements.
To apply the waterproofing spray, spread the item you are spraying out in a well ventilated area. Make sure that there is no wind and that the weather will stay good long enough for the spray to dry if you are spraying the canvas outdoors. Use long, side-to-side overlapping strokes to help ensure that the entire piece of canvas is coated. You may also want to apply a second coat after the first to be sure that you didn’t miss a spot.
Be sure to get the edges of tents and tarps well. You may want to consider flipping the edges up and spraying the underside as well for added protection. With lawn furniture, remove all the cushions and spray both the top and bottom of each to provide maximum protection and to help the fibers of the canvas last longer.
Canvas that is used regularly outdoors should be waterproofed at least once a season. Remember that no waterproofing method will last forever, so keep an eye on your outdoor items and make sure that the water is still beading up on them when they get wet. If not, it is probably time to spray again.