If you've noticed that your boat's canvas cover or bimini top is developing mould, and water is frequently dripping through the fabric, you may need to apply a new waterproof coating to the canvas. Your boat's covers are likely made from acrylic canvas, due to its mould-resistance and breathability. Acrylic canvas requires a water-repellent coating. In contrast, cotton canvas, though susceptible to mould, is waterproof by virtue of the fabric's tendency to swell, which impedes water flow. Acrylic canvas can lose its water repellent coating from wear, and may need a yearly coating of a water-repellent chemical, depending on the conditions into which it is placed.
Clean the canvas using dish detergent and a scrub brush. Rinse the canvas thoroughly, and allow it to dry.
Spread the canvas out on a flat surface. Spray the canvas thoroughly with the water repellent, noting the direction of spray in relation to the canvas. Spray a second coat on the canvas in a direction perpendicular to the first coat. Allow the canvas to dry for a duration specified in the water-repellent directions.
Reinstall the canvas for use, once it has dried completely. The canvas should remain waterproof for at least a year, depending on the frequency of use.
Protect plastic windows sewn into the canvas, using tin foil sheets and masking tape. Cover zippers with masking tape. Do not use a silicone-based water repellent on acrylic canvas.