Help! Cast iron bath stains??!

Ive got an old cast iron bath thats got stains on it from when we moved in. Ive used cif, bleach, cillic bang all with both cloths and scaring pads! those magic sponges that are supposed to remove anything. Nothing works ive got to the point where i just want to break it somehow so i can get a new one from the landlord? Can anyone help please??