How do I get rust off a tub?

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 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.

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.

i would just clean and re-glaze