Is the liner still there? Is that what is stained? Is it a whole tub liner, that covers up the entire tub? What is it made of?
Here you go!
When I had my pink tub refinished years ago they told me to use only original scrubbing bubbles on it to keep the 7 year warranty. No bleach or abrasives. Also Mr. Clean Magic Erasers work well without scratching.
Hi Tamara, if you click on "explore projects" then on "clean" you can find many ways suggested that others used to clean their tubs with liners. I typed "clean tub" in the search box too and found more. I chose one for you, but you might like other ideas also. Good luck on this! https://www.hometalk.com/18973805/diy-3-ingredient-tub-and-tile-cleaner