If you have a plastic item with rust stains, they aren't there because the plastic is rusting. The plastic has been in contact with rusting metal -- it could be the bottom of a chair with rusting metal legs or a plastic bucket that held rusting nails. Moreover, PVC plastic fencing can show rust stains left by iron in rainwater; the iron oxidizes and discolors the plastic. You don't have to worry about neutralizing the rust; you can simply remove it by scrubbing it or dissolving it with an acid. If using an acid sounds extreme, don't worry. The acids you need for the job are so safe you can ingest them -- and you probably do.
Household Acids for Rust Removal
Rust is iron oxide, and it will dissolve into an acidic solution, even a weak one. Both vinegar and lemon juice are acidic; vinegar contains acetic acid, and lemon juice contains citric acid. You could also clean the rust with any tangy soft drink; the tanginess is caused by the presence of phosphoric acid in the drink. Soft drinks don't make the best cleaners, though, because they leave sticky sugar residue.
Start with the gentlest cleaning method and progress to stronger ones for plastic furniture and other indoor items. You may be able to remove most of the rust using nothing more than a wet rag. If you're cleaning outdoor PVC that has been discolored by exposure to rainwater, it's best to jump to the strongest cleaning method possible.
Moisten a rag with water and rub the rusted area vigorously. If the stain doesn't come out, add a little dish soap to the rag.
Increase your scrubbing power by exchanging the rag for very fine steel wool. If you're worried about dulling the finish of the plastic, use a kitchen scrubber instead. Wet the steel wool or scrubber down, add a bit of dish soap, and scrub away.
Mix a 50-50 solution of vinegar or lemon juice and water to remove stubborn rust stains. Put the solution in a spray bottle and spray the rusted area liberally. Let the solution work for five to 10 minutes, the wipe it off. Repeat if necessary.
Use full-strength vinegar or lemon juice to remove stubborn rust stains. Spray it on, let it work for 10 minutes, and wipe it off, repeating as many times as necessary. Use this method to clean rust from exterior plastic, such as PVC fencing, vinyl siding or PVC decking.
Make a paste to handle very stubborn stains. Mix full-strength vinegar or lemon juice with borax, spread the paste on the stain and leave it overnight. Scrub the paste and rust off with a scrubbing pad in the morning.