Iron out works great! You can buy liquid form or you can get the powder version and mix it yourself. Soak it overnight. Use toothbrushes or anything with tough bristles to get anything loose off first. You can find Iron out in just about any plumbing section of a store.
Try adding an acid like vinegar to a bucket of quartz with hot water. Clean with tooth brush. That's what i did but it takes a while.
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