Acids are what's used to etch porcelain bathtubs for "refinishing"
with epoxy paints. Even leaving chlorine bleach solution puddled
on the tub surface will etch it -- I found that out the hard way.

Which acid or acids? Decades before I saw "Breaking Bad", I knew
that hydrofluoric acid attacked glass and porcelain. But hydrochloric?

I recently discovered that caustic soda in an ultrasonic cleaner
is capable of etching the glaze off spark plug insulators.

Okay, but we're talking about acids directly applied to porcelain.