Get a sheet of wet/dry sandpaper, the finest grit they have. Clean the toilet as ususl and then using a piece of the wet sandpaper, with a gentle touch, "sand" the areas that have the hard water stains. This type of sandpaper is sold everywhere regular sandpaper is. Wal-mart, K-mart, Lowes, etc. Works beautifully and no chemicals.

Also can use a pumice stone if sandpaper isn't enough. Available at hardware stores.

Use a pumice stick.

Here is a link that might be useful: Pumice Stick

See if you can find ZAP. It should come with a scrub pad. It's the only thing I found to remove the stain. It will also take a little elbow grease.

Have you had any laboratory instruction in handling harsh chemicals? If you have had instruction in how to handle it you can get sulfuric acid or muratic acid at large hardware stores. Both are quite nasty unless you've had instruction.

We live in one of the many suburbs of the southwest where boron in the water is a problem. My spouse prefers pumice. I use the wet "sandpaper" mentioned if accumulation is not to bad. In sever situations I use sulfuric.

I saw a pumice stick like the one graywings linked to at Walmart today...hanging on and end cap display. I think that would work nicely.

This worked for me. Flush the toilet and turn it off right away. This will empty it most of the way. Empty the rest of the water out. Fill with full strenght vinegar and let sit for a couple of hours. That will get hard water deposits and other deposits out.

A Magic Eraser works well, too. In fact a Magic Eraser got my hopelessly stained bathtub clean.

I swear by Barkeeper's Friend. It removed rust stains in a toilet that nothing else would remove.

You might try those effervescent denture tablets. They supposedly work well on this kind of stuff. (have heard this, don't know from personal experience. fortunately, there is enough chlorine in our water that the toilets never get stained.) Don't know how many you would need though.

I got to thinking about this yesterday while cleaning the toilet.

Toilets have changed over the last couple of decades so and the sort of toilet one has now may make a difference.

For example mine is a 1983 American Standard. I can just dump a couple of gallons of water in it and it will flush and not refill. Then that old surface will withstand pumice or strong acids or automotive polishing materials.

Will all (Any?) of the more modern models tolerate the things mentioned in my previous paragraph?

I've seen my favorite Queens of Clean saturate paper towel with lemon juice and "wallpaper" the cruddy areas with it. Let it soak for 30 minutes or so and the citric acid in the lemon juice will loosen up the junk. Then they used a pumice stone to dislodge the rest.

When we moved here the toilets were stained. I flushed the toilet to get the bowl wet, and turned off the water. Then I soaked heavy paper towels in Lime-Away and covered the stains with the towels. I let them sit for 24 hours (we hadn't moved in yet, so don't get excited!) and the stains were gone. This method would probably work with chlorine bleach, Barkeeper's or any other liquid cleaner. I wouldn't recommend using anything that will scratch the porcelain.

Don't use the Clorox (or anything with bleach) while you're using the products with acid - it produces toxic fumes. The bleach won't get the mineral deposits off. The Lime-Away, The Works, lemon juice, vinegar, anything with acid or Barkeeper's friend will work much better. Whatever you use, get as much water out of the bowl as you can before you start so you aren't diluting the product too much and let it set for some time before you try to clean. (The worse the stain, the longer it will need to set. I've had to leave some set 24hrs the first time and then another couple hours each of the next two applications (VERY stained toilet!)