Vinyl is one of the most common plastics made and has a wide variety of uses, including in swimming pools. Since all vinyl is virtually the same, it will respond well to any cleaner specifically made to clean vinyl, be it a commercial brand or a homemade type. You have a variety of methods to choose from, then, for removing rust stains from your vinyl swimming pool.
Sprinkle some baking soda onto the rust stain. Wet your rag with pure white vinegar and rub it over stain. You will notice the rust stain begin to fade with each pass. Sprinkle more baking soda over the stain and wipe again after freshly wetting the rag with vinegar. Continue this application until the stain has disappeared.
Mix reconstituted lemon juice with salt and turn it into a slurry. Dab or spread this concoction onto the rust stain and allow it to sit for several hours or until the mixture turns dark orange. The dark orange coloration means the rust stain has been lifted, and you can then rinse with fresh hot water.
Apply a paste of cream of tartar mixed with water and allow it to sit for approximately one hour. Take your scrub brush and scrub the paste from the vinyl until the rust stain disappears.
Clean the stain away with a commercially made vinyl cleaner that contains 5 percent oxalic acid. Wipe the cleaner on, wait a few minutes, and then wipe it off.