On 2013-04-09, EXT wrote:

Had this problem with my grown son who didn't bother to flush nor clean.
Nothing would remove it, so after draining as much water as possible, I
poured in about a cup or so of acid pipe cleaner from Home Depot that I had
sitting around. When it was poured in it foamed up, got hot, and then I ran
an all-plastic scrub brush around in the bowl. The liquid turned dark brown
and when I flushed it down it left a clean, stain free toilet.

I have some sulfuric acid drain cleaner. Would that harm porcelain?
I called the company that makes it and asked them if it would hurt
plastic tub drain piping. They would commit/deny/agree with
absolutely nothing. Kinda like, "Hey, we jes make it and ain't
'fessin' to diddly. Use at yer own risk". Real encouraging.

nb