Muratic acid will etch porcelain. I bought a cast iron clawfoot tub that had some mean rust stains inside. Nothing worked(CLR, white vinegar, dilliuted bleach ), so I got out the Muratic.....it worked, but it ate the surface of the porcelain leaving it dull and now porous. We used the tub for a few years but that etched area attracted dirt like crazy, ended up buying a brand new 6' Iron Clawfoot from chi-na....the thing is a beauty.
I only use white vinegar to clean the toilets and we have some heavy mineral well water here....they always come clean.
1996 Fleetwood Flair 22' (aka Stumpy) Chevy 454