I use a Pumice Stone and Comet to get any and all stains out of the bowl. I have not had a Pumice Stone scratch my bowl.
Use a wet pumice stone. You can find them in hardware stores and grocery stores. Works great!
Watkins sells an acid cleaner that will do it. I got battery acid at an auto supply store. It also cleaned the toilet from an old house. You need lots of ventilation for that.
Comet with bleach. Just before you leave, with your toilet brush, push the water down to the bottom of the tank. Just keep stuffing the water down so that the tank is mostly empty except at the bottom. While sides are still damp (or you spray/sprinkle water), try good old comet with bleach. If sides of tank are wet, comet will stick. Just leave sit, or sprinkle/spray with water to activate the bleach. You can leave it there for a week.
White is beautiful in a toilet.
Have you tried a pumice stone? I would try that first. If it has been there that long cleaning solutions aren't going to touch it.
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