Vinegar, bleach, lemon juice, salt all doesnt work
I really like Barkeeper's Friend spray for hard rust stains. The trick is to spray and let sit for 5 minutes and then scrub. If you don't let it sit, it doesn't work as well.
Have powderived does sort of but never seen spray
Have you tried CLR.?
If you have a box of borax for laundry, a bit of that may help. Sprinkle a bit on a sponge a nd give it a try.
That may require professional help with either grinding tools or chemicals.
What's your tub made of? Acrylic, fiberglass or cast iron?
I agree with using Bar Keeper's Friend. You can make a paste out it with water and let it set for 15 minutes before scrubbing. The oxalic acid in BKF is what helps to remove the rust.
Lysol Lime & Rust Remover Toilet Bowl cleaner, comes in a black squirt bottle. Scrub with Scrub Daddy or Mr. Clean Magic Eraser.