MadEngineer:
yeah you've misinterpreted. i get that it's pee stains, my question is how it even became stained by pee - porcelain should have a good solid clear coat that shouldn't be stainable unless it's been damaged, perhaps by overcleaning (think like teeth)
Fair enough. We had a kid here who left pee stains, hard to remove but easy with bicarb. Not really stains as you say, its just gunk that sticks. Like any awkward cleaning issues, its finding what reacts with, and dissolves the substance.