Thanks all! I found all the answers I needed - drywall sand paper did the trick. Pretreating with lime-away may have helped (not sure but was done prior to scrubbing). I don't see any scratching on the porcelain.

I wouldn't use anything abrasive. When we lived on a boat I discovered that vinegar (get the cheapest white vinegar you can find) will take care of stains in the head (aka marine toilet). Empty the bowl, pour in the vinegar and let it sit, might take a day.

Once clean, I don't think you will have any problem with stains. I am 75 and never had a problem. You just have to clean them often and the ones rarely used need to be flushed occasionally and cleaned.

I try to keep the use of chemicals to a bare minimum, so I use soap and water for most cleaning. I use cheap dish washing liquid to clean the stools and have been doing that for 2 or 3 years. If I buy shampoo that I don't like I use it to clean the stool. I haven't seen any problems doing that. My neighbor said, "but it's not sanitary", my reply was I don't drink out of it. LOL

you are right to use some abrasive scrubbing item such as a pumice stone or what you ended up using. My mother said the only thing that works at her house since they have such hard water is to empty the toilet of water to get at the ring and then use an abrasive sponge or pumice stone to go after it. She says it's the only thing that works. Her home is spotless, it's not that it's from not cleaning it's something to do with her water. I've seen this discussed before and in the end it's the only way that seemed to work according to others who had the same problem.