my 2 cents. I don't like the m. acid - never worked on really tough/deep dark porcelain stains for me yet - don't breath that stuff at all like already stated. Search on this site.. good info.. from past on this. Try mother's chrome polish (drugmart, kmart) or Weidmans stove top cleaner and green scratch pads -- takes time and work but does the job. Couple times for me on a really tough porcelain stain not even m. acid would cut it when these other 2 did nothing - m. acid was my last resort and nil affect. Just what I have experienced. be safe.