Use a waterproofing product that is petroleum based. As Casey points out, "Waterproofers that have a petroleum-based vehicle have demonstrated better performance on acrylic canvas than water-based treatments. You can identify a petroleum-based product by its strong odor."
Remove the canvas from the boat, take it outside and spread it out, flat. Clean the canvas with a mild detergent and scrub brush and allow the canvas to dry completely.
Spray the waterproofing product on the canvas using a vertical (up-and-down) motion. Allow the waterproofing to dry completely on the canvas. Avoid any "windows" and plastic zippers on the canvas, since these may be damaged by the waterproofing compound.
Spray a second coating of the waterproofing product on the canvas using a horizontal motion--side-to-side. As Casey says, "This 'plaid' pattern tends to yield more uniform coverage."