What can I use to clean tough bathtub stains ? ?

I have tried bar keepers helper, vinegar, tilex. I have tough bathtub stains on the bottom of my bathtub that won't come off. It's on the bottom where the groves are. These cleaners have helped lighten the black stains but they are not coming off completely. I rent and am moving soon and want to get my deposit back. Any suggestions on what I can do ?