Rinse any loose dust and dirt from the outdoor plastic with a water hose that has a sprayer. Wear rubber gloves and an apron or other cover-up to keep from getting wet.
Mix 1 cup of dishwashing soap, 1 cup of chlorine bleach (if the outdoor plastic is colored, use white vinegar instead of the chlorine bleach) and 1 cup of warm water in a bucket. Agitate the mixture with a long-handled brush.
Pour the soap solution into a spray bottle. Use a funnel to keep from spilling the solution. Not all of the solution will fit into the spray bottle.
Spray the soap solution on any stains and spots present on the outdoor plastic.
Let the soap solution set for 10 minutes.
Dip a scrub brush into the bucket of soap solution and apply the mixture to the outdoor plastic surfaces.
Scrub the outdoor plastic with the brush to break up and remove any dirt, dust and grime buildup. Use a sponge on areas where the scrub brush can't reach.
Rinse the outdoor plastic thoroughly with a water hose.
Let the outdoor plastic dry in direct sunlight.