The Washington Post recommends cleanning plastic furniture thoroughly before use in the spring and then as needed throughout summer and fall, after harsh weather or animal damage. Clean the furniture in phases. First, sweep away any debris, such as leaves. Next, remove dirt stuck to the furniture. Even out any rough areas, and apply cleaning solutions to any stains or discolorations. Finally, polish the furniture to finish.
For the initial cleaning of loose debris, simply brush down or vacuum the seat, tabletop and legs of the furniture. Create a soap-water solution by mixing 1 gallon of water with 1/2 cup of clear soap. Apply the soap-water mixture using a sponge, and rinse with water from a low-pressure hose. Avoid using a hose with high water pressure as that can damage the furniture. Use warm water but not hot water.
If stains remain after the initial washing, The Washington Post recommends applying a mixture of water and bleach containing sodium percarbonate. A good ratio is 1 cup of bleach per gallon of water. After the furniture dries completely, smooth and shine the plastic with polish containing paste wax, such as car polish, to repel dirt. If the furniture still looks stained or discolored, consider sanding and repainting it.