Last night I somehow dreamed I had a hydrofluoric acid burn. Reading this thread I remembered there was this rust stain remover product on US Amazon that is 3% HF. https://www.amazon.com/product-reviews/B000LNQNM0/
It's got great reviews (and I'm inclined to believe that this product will do the job). There's one 1-star review that stands out though: "Simply put. It doesn’t work and it was shipped leaking." Yikes!
It's got great reviews (and I'm inclined to believe that this product will do the job). There's one 1-star review that stands out though: "Simply put. It doesn’t work and it was shipped leaking." Yikes!