Before canvas is cleaned it should be dried in a warm, sunny location, and have any loose debris removed with a soft brush. Small items, such as canvas bags, can be cleaned using equal parts rubbing alcohol and water applied with a clean cloth. The item should be rinsed thoroughly with cold water immediately after cleaning.
Borax works well to treat larger mildew stains. If the item can be washed, 1 cup of Borax can be dissolved in a gallon of hot water and added to the wash cycle. Items that cannot be washed, such as seat cushions, can be cleaned with a mixture of 1 tablespoon of liquid dish detergent and 1 tablespoon of Borax dissolved in a quart of warm water. Apply the solution with a soft brush, and then rinse thoroughly with more warm water. The canvas should then be placed in a warm, sunny location to dry, with windows and doors left open to ensure the area has proper ventilation while the material is drying.
Undiluted white vinegar kills mildew and removes odors on canvas. Use a spray bottle to apply the vinegar on the affected areas. A clean, dry cloth will remove the mildew after the canvas has been treated.Learn more about Cleaning