While a canvas backpack is extremely rugged and sturdy, able to withstand excessive abuse and harsh environments, it can begin to look soiled and dirty over time. Many canvas backpacks have a special coating that creates a waterproof barrier to prevent backpack contents from water damage. After you wash a canvas backpack, reproof the canvas fabric to renew the waterproof-repellent capabilities of the canvas and protect your gear from water.
Saturate the nylon brush with water and scrub at the outer surfaces of the canvas with the brush. Work the brush firmly onto the surface of the canvas to loosen soiling and stains. Blot at stains and water on the surface of the backpack with a towel to remove the water from the backpack.
Mix approximately 1 tsp. of laundry detergent with 2 cups of warm water in the small bowl. Dip the nylon brush into the soapy water and use the soap-filled brush to remove any stubborn stains from the outside of the backpack. Continue working until you remove as much soiling and stains as possible. Saturate a cloth with warm water and use it to rinse away the mild soap from the backpack canvas.
Spray the dry-cleaning solvent carefully over any stains that remain. Place a towel on the underside of the backpack fabric to absorb excess solvent as you apply it. Saturate the stain with the solvent and allow the solvent to soak into the canvas for approximately five minutes. Blot at the stain with a clean cloth to remove the stain completely. Rinse away the dry-cleaning solvent with a damp cloth soaked in water.
Allow the backpack to dry completely (outdoors in the sunshine, preferably).
Reproof the backpack to restore its water resistance. Fill the inside of the backpack with a large bath towel to absorb any spray that seeps through the backpack. Zip the backpack closed. Spray the canvas proofing spray liberally over the entire outside surface of the backpack. Pay special attention to seams, zippers and pockets to ensure you coat every surface.
Allow the backpack to dry again for 3 to 5 days before using it.