The sunlight has natural bleaching power that will fade plastic without the use of chemicals.
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Place the plastic item in a sunny window if the weather doesn't permit it to be outside. Leave the item in the window until it has faded as much as you want. It may take a few days indoors. Rotate the item a couple times a day to even the fade.
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Put the plastic outside in full sunshine. Light fading will occur within a few hours. If the plastic is designed for outdoor use, it may take a few weeks for you to see a noticeable difference in the color.
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Remove stains on plastic by putting it outside for the sun to bleach. This method works particularly well for tough tomato stains on food storage containers.