It doesn’t matter if you have plastic or metal outdoor furniture, you can rid the grime they collect with cream of tartar. Mix equal parts of the cream of tartar with lemon juice OR white vinegar. Apply to the furniture. Let this sit in the sun for about 5 minutes then rinse off with the hose. The cream of tartar will have reacted with the vinegar or lemon juice to lift off stubborn gunk and grime on plastic while it also will break down any rust on metal. Don’t use on wood furniture. Instead, make the paste using only water and cream of tartar to clean.