Things You'll Need
Washing soda or baking soda
Outdoor plastic furniture can become stained black from the minerals in rainwater, dirt and grease among other things. It may be tempting to just throw out and replace stained plastic lawn furniture, but it can be cleaned with a few common household items. The trick to getting rid of black stains is to use something that will lighten the stain without being harmful to your outdoor environment or further damaging the furniture.
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Hose down the plastic lawn chairs to remove loose dirt and debris. Put on a pair of rubber gloves
Fill a large bucket with 1 gallon of water. Add 1/2 cup of washing soda or 1 cup baking soda. Washing soda is stronger so you don't need as much of it. Allow the ingredients to dissolve.
Dip a long handle scrub brush into the bucket and scrub the chairs. Dip the brush frequently to guarantee you have enough cleaning solution on it.
Keep the cleaning mixture on the chairs for 10 minutes. Rinse clean with a hose.
Check the chairs. If there are still black stains, put some washing soda or baking soda on a clean moist sponge and scrub directly on the stain.
Let the pasty film from the cleaner dry on the stain. Rinse clean with the hose.