How to remove stains from a bathtub caused by dran-o?

[brown_chair]

Hi everyone. I live in a really old apartment building and poured some draon-o down my bathtub, and several minutes later this stain appeared on the bathtub. If I had to guess it's the rust from the pipes. I've tried baking soda/white vinegar, more drain, bleach and CLR with absolutely no budging of this stain. Any help would be appreciated!

 

[havasu]

I'd try Barkeeper's Friend, and try to give it a slightly abrasive rub down.