Before getting into IT, I owned a 13-man building cleaning and maintenance company. Part of our work was janitorial. We used commercial bowl cleaner with high hydrochloric acid content. If we ran out and couldn't get to the commercial supply place, Sam's Club had something comparable, but a bit weaker. Hydrochloric acid will burn your skin off, but it also eats off sludge, grime, calcium, etc. and your toilet will look like new. It will eat everything down to clear porcelain. Just be sure to rinse thoroughly afterwards - it's highly corrosive. I believe there are also some household bowl cleaners (Lysol, etc.) that have hydrochloric, but they're even weaker than the stuff Sam's used to carry.

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