I have used WD40, baking soda and vinegar.
Lillian....Have you tried CLR?
Hi Lillian, I have always used the works toilet bowl cleaner and it even works on built up stains. You may have to do it a couple of times. Put it in and let it sit for ab out 10-15 minutes before you scrub. Do not buy the one that says basic on it as it is a really cheap version that does not work.
CLR is amazing, I use it in my toilet bowl for rust deposits, available at diy stores.
Lysol Lime & Rust Remover Toilet Bowl cleaner, comes in a black squirt bottle. Scrub with Scrub Daddy or Mr. Clean Magic Eraser.