Rust develops on metal objects when exposed to moisture for a prolonged period of time. Rust won't normally develop on plastic windows but it can transfer to the surface when it comes in contact with a rusty item. Removing the rust stain on your window is important as it can easily transfer to curtains, in addition to taking away from your window's appearance and functionality.
Combine 1 tbsp each of baking soda and lemon juice in a bowl. Stir the two ingredients together with an old toothbrush to create a rust-removing paste.
Spread the paste over the rust stains on your plastic windows, using the toothbrush. Allow the lemon and baking soda paste to sit on the stain for 10 minutes.
Dampen the old toothbrush and scrub the lemon paste into the rust stain. Rinse the brush with water, apply more paste and continue to scrub to remove the rust stain.
Wipe off the lemon and baking soda paste with a clean cloth. Spray window cleaner on your window, then wash to remove any paste residue.
Wipe off a wet rust transfer stain as soon as it's noticed to prevent staining.
When using commercial rust removers be careful to use a product that won't damage your plastic windows. Avoid abrasive cleaners on plastic windows as they scratch easily.