First, clean any dirt or dust off the surface of your rubber with a soft brush or cloth. Then, mix together equal parts of water and bleach in a spray bottle or bucket. Spray or pour this mixture onto your rubber and wait for about five minutes for it to soak into the surface. Then, use a sponge or rag to scrub off all of the mold and limescale residues from your rubber. Rinse with clean water and dry completely before using again!