You'll want something that can dissolve iron oxide, generally a weak acid. Products like CLR or homemade vinegar (white vinegar) solutions or some of the household cleaners that have citrus (citric acid). One problem you may encounter is the porosity of plastic--ever reheat spaghetti sauce in Tupperware? That's the problem. If it doesn't come off with a cleaner, you can try to clean it hot (in the sun all day with hot cleaning solution). This will swell the plastic and hopefully let the cleaner into the pours of the plastic. Otherwise a repaint would cover it.
good luck!