Things You Will Need
A soft bristle brush, possibly an old tooth brush to get into tight areas, sponge, water hose, Depending on how bad the mildew stains are one or more of the following: Liquid Ivory soap or another mild neutral ph detergent, a solution of 50/50 water and isopropyl (rubbing) alcohol, 1/2 cup Lysol to a gallon of water or a cleaning product such as ISSO. If stains are persistent, a mixture of a non-chlorine bleach such as Clorox II in weak solution with water or as a last resort regular bleach (which can turn canvas yellow) or mildew products such as Tilex or X-14.
Brush off any loose dirt and hose the fabric with water.
Mix one of the solutions mentioned above such as the Ivory soap with water (1/4 cup soap to 1 gallon water) and sponge into the stained area and let soak for several minutes.
Scrub the canvas with the soft bristle brush while keeping the canvas wet with the soap solution. Then rinse thoroughly.
If stains remain after the initial cleaning, it will be necessary to use one of the other mixtures or products listed above. Begin by mixing the Clorox II and water and repeating Step Three above. If the stains are still present it may be necessary to use regular bleach, Tilex or X-14 to clean the area. I would do a small space to test the fabric for color fastness before cleaning the whole top or cover. If the color holds, repeat Step Three again.
You can help prevent future growth of mold and mildew by spraying on a mixture of 50% white vinegar and 50% water. A garden pump sprayer is good for this. Vinegar will not harm or stain the canvas.
The cleaning process will probably damage the waterproof finish of the fabric. After the fabric has completely dried, treat the fabric with products such as Aqua-Tite, Camp-Dry or 303 High Tech Fabric Guard to restore the water repellency and to extend the life of the fabric. Spray on one or two coats of the treatment allowing the first coat to dry completely before applying the second coat.
Mold and mildew on canvas boat tops and covers look bad and can be potentially unhealthy. With the right cleaning tips and a little work the boat top or cover can be cleaned to look new.
Tips & Warnings
Start with the mildest soap solutions first to see if they will successfully remove the stains before using the stronger solutions. Wear gloves and safety goggles when working with the stronger solutions to protect your skin and eyes
Bleach and other chemicals used to clean mold and mildew can be poison and the vapors can be toxic to individuals with breathing problems. Some individuals also may have allergies that can be aggravated by the mold and mildew itself.