In article , "Frank Stacey" f.
says...
During recent renovations to one of our bathrooms, the outside of the WC
bowl has become sprayed with some black dust-like muck which is proving
difficult to remove. We have tried cleaning off with typical bathroom
cleaners but the stuff is reluctant to be come off. What is the most
abrasive "thing" we can apply to a ceramic surface without damaging the
surface itself?

[The origin of this mess is a not known. In the early stages of renovation
I had to remove on old metal shower cabinet which was so "well" built in
that I used an angle grinder with a cutting disc to break up parts of it.
This sprayed some sparks (hot metal) around and looks the most likely
culprit. Only the ceramic has this mess on it. Formica and plaster surfaces
are as clean as ever.]

The black specks are bits of grinding disk and steel that have welded
themselves to the soft ceramic glaze - the only thing I can think of
that might improve the appearance is treating it with some sort of acid
to dissolve the iron component (which will otherwise rust and cause
staining) but I suspect that won't remove all the marks, and even if it
did you'd be left with the little craters that have been burned.