In article ,
The Natural Philosopher writes:

The rim wasnt too bad, but round the bend was evil. It was about half
the diameter it should have been and not JUST scale., Layers of scale ad
****. Years of scale and ****. Took about a week of acid, chipping, and
flushing for an hour after work each day, to clean it all up.

Acid isn't too effective on the **** part. What you can do
in this case is to alternate it with another powerful cleaner,
such as washing machine or dishwasher detergent dissolved in
hot water and left in the trap over night, which are both good
at clearing the organic debris.

When switching cleaning agents, flush the previous one well
away first, enough times to clear the sewage pipework too.
Mixing of different cleaners can cause violent chemical
reactions and production of toxic fumes.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]