the thing you have to do is figure out what the stain is. Is it a stain from the water or did someone use a stainless steel scrubbing brush? If it is from a stainless steel scouring pad or brush then the gray stain is from actual bits of metal ground away from by the very fine pores in the surface of the glaze. You would have to use something very powerful to remove it. Probably Muriatic Acid. They sell it at the hardware store. It's a drastic measure and may cause problems if not done properly.
that being said, 40 years in toilet years is like 32 years in dog years which would be about 197 years in people years. It's really way past time for a new toilet.
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