It can (does) leave a little bit of a burn mark in the
immediate area of the weld, but that is relatively easily
cleaned... especially compared to boric acid, flux, solder,
pickle, buff, polish, etc.
I’ve heard (never tried it, don’t have a Sparkie) that putting a
drop of distilled water on the flat part (where the nib will
contact it) will eliminate the burn mark. Maybe someone could