On Friday, July 21, 2017 at 11:44:09 AM UTC-4, Mike wrote:
replying to Robert Macy, Mike wrote:
is this like muriatic acid?

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No, he's talking about hydrofluoric acid, a different acid, which
will etch glass. I wouldn't use it to clean a stain in a bathtub.
I would use muriatic acid which is hydrochloric acid. Or a rust
remover product like CLR.