Things You Will Need
A soft bristle brush, possibly an old tooth brush to get into tight areas, sponge, water hose, plastic tarp to cover plants if necessary, and a ladder to reach the awnings. 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.
Some plants can be damaged by the soap or other chemicals you may have to use in cleaning the awnings so it is usually best to wet them thoroughly and cover them with the plastic tarp while cleaning above them.
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 your soft bristle brush while keeping your canvas wet with the soap solution. Then rinse thoroughly.
If stains remain after the initial cleaning, and they probably will depending on the severity of the stain to begin with. 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 Four 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 awning. If the color holds, repeat Step Four 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, retreat 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 awnings can be both unsightly and potentially unhealthy. It can also have a negative impact on curb appeal of your property so use the simple steps above to clean them and make them look new again.
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. If the awnings can be removed from the building it is easier and safer to clean them on the ground then standing on a ladder.
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.