Fill up a tub or bucket with cold water and soak the oilskin duster. Alternatively, hang the duster outside or in the shower and spray it down with cold water.
Scrub the duster with a stiff scrub brush to remove any dirt or debris. If there is any mildew, apply a solution of 1 part vinegar to 3 parts water, and scrub it well. Rinse the duster thoroughly with cold water.
Hang the duster in a warm, dry room or outside away from direct sunlight until it's fully dry.
Place the duster in a sunny location to warm up the fabric slightly, or warm it with a hair dryer on low heat. Do not leave it for too long, as the sunlight may bleach the fabric. Warm up a closed jar of reproofer in some warm water. Apply a small amount of reproofer to the duster with a soft cloth, focusing on the seams and friction points. Remove any excess reproofer with the cloth.