Yellow stains on toilet seats can be caused by urine, rust or hard water or they might be caused by the use of bleach. If the stains have developed from bleach, you may be unable to clean them completely but they could possibly be lightened.
Wear rubber gloves. Fill a bucket with water and set it aside.
Carefully pour a small amount of the hydrochloric-acid-based cleaner on a nylon-bristled brush, holding it over the open toilet bowl.
Scrub the brush into the stains on the toilet seat slowly and carefully. Clean up any spatters on the floor or your skin or clothing immediately with a wet cloth. Use the bucket of water to wet the cloth. Reapply cleaning solution as needed.
Rinse the toilet seat by running a wet cloth over the surface.
Dry the toilet seat completely with a clean cloth.
A hydrochloric-acid-based cleaner may list diluted hydrochloric acid, hydrogen chloride, HCI or muriatic acid in its ingredients.