The type of canvas used for tents, awnings, camper covers and other outdoor applications usually contains natural fibers such as cotton. Unfortunately, this means the fabric is an ideal breeding ground for opportunistic fungi and molds. While many canvas-based products are treated with chemical water repellants, this does not mean they are resistant to fungal growth, especially during long storage. There are a number of ways to remove fungus from canvas surfaces, but the threat of re-contamination is always present.
One reason why it is so difficult to remove fungus from canvas is the nature of the fungus itself. Fungus spores become airborne as they dry, and the wind drives them to other surfaces. If a fungus comes in contact with both moisture and a natural food source, such as cotton fibers in canvas, it will produce more spores in all directions. Only a sustained lack of moisture will stunt the growth of fungus, but even then the spores only become dormant, not dead. Once the dried fungus spores encounter moisture again, they often revive and thrive.
To remove fungus from an outdoor canvas, some experts suggest drying the material completely, whether by exposing it to the sun or using a mechanical dryer. Once the canvas is completely dry, the owner should put on protective gear such as gloves, face mask and safety goggles. Most fungi are not especially harmful to healthy people, but some do contain poisons called mycotoxins. Use a stiff brush or broom to agitate and sweep away as much of the surface fungus as possible. Be sure to protect other non-contaminated objects through isolation or protective covers.
After the initial layer of dry fungus has been swept away, there are a number of ways to remove fungus remaining in the fibers of the canvas. This is where opinions differ, so you may want to do some research before settling on a plan of attack. Some sources suggest using an oil soap solution and a stiff brush to remove the fungus. Others caution against using commercial dishwashing soaps or detergents, because they can remove the natural color of the canvas. Anti-fungal products containing bleach, often used to remove mildew from tubs and tiling, could also be used to remove fungus from canvas.
The most recommended way to remove fungus from canvas is a diluted mixture of chlorine bleach and water. This method actually kills fungi and other molds on contact. The canvas should be treated with the bleach mixture and then rinsed thoroughly. There may be some staining or color removal caused by the bleach, however, so owners should consider testing a small patch before committing to this method. Bleach will also remove other chemical treatments such as waterproofing, so the canvas may have to be re-treated after it dries completely.