Cotton is a fabric that generally absorbs water and becomes saturated and damaged with too much exposure. Knowing this, you may wonder how cotton jackets, outdoor upholstery, boat covers and other items maintain their appearance even after being soaked many times. Products such as these, which are intended to withstand moisture, usually are treated with waterproofer during the manufacturing process. Fortunately, you can purchase spray-on waterproofer in a variety of formulations and transform almost any cotton item for outdoor use.
Identify the type of cotton fabric with which you are working. Cotton canvas and even denim may be pretreated with a fluorocarbon protectant. You must use a waterproofer with a compatible base, such as a fluoropolymer; all other basic cottons can be treated with a silicone-based waterproofer.
Wash the cotton with mild detergent and dry it thoroughly on a low setting. Scrub tarps or other items too large for your machine with a nylon brush and a mixture of detergent and water. Rinse these large items with plain water until no soap remains and let them air-dry.
Cover zippers and other hardware completely with masking tape. Make sure waterproofer cannot get on these items, or they can be damaged. Lay the cotton on a flat surface outdoors and spread it out as much as possible.
Spray on one thin coat of waterproofer in even lines going up and down. Cover the entire cotton piece in one direction. Move so that you are standing perpendicular to your original position and spray on a second thin coat in the other direction.
Let the waterproofer dry 2 to 3 hours, unless otherwise directed by your particular product. Turn the cotton over and repeat the same steps with opposite side. Let the cotton dry completely and remove any masking tape.