Clean waterproof canvas with a solution of 1/4 cup of liquid dish detergent dissolved in a gallon of warm water. Work the water into the canvas with a soft brush, rinse with warm water and allow the canvas to completely dry. Alternately, wash small items in the washing machine set on warm with a mild detergent.
Waterproof canvas is often used on upholstered outdoor furniture, other outdoor patio accessories and sturdy beach bags. If the fabric is removable and fits in your washing machine, that is the preferred cleaning method. Do not dry waterproof canvas in the dryer. Instead, allow it to air dry. If the canvas needs to be reattached to a frame or refilled with a cushion insert, complete those steps while the canvas is still partially wet. This helps the canvas to stretch and conform to the proper shape better as it dries.
Mildew stains can usually be removed with a solution of 1 cup of regular household bleach mixed with a gallon of hot water. Test a small area for colorfastness before doing the whole item. Rub the stained area with the bleach solution. Let it sit until the mildew stains disappear but no longer than 15 minutes. Rinse well with warm water.