That is a $30 piece of sill marble, much cheaper to just replace,I'm betting you find rusty roofing nails tacking the pan liner on top of curb, "poor installation method"
My opinion is you may lighten but never remove these stains
Best Regards
Scott Wilson

Sent from my iPad

> On Sep 18, 2017, at 8:54 AM, Mike Alston <realmikealston@gmail.com> wrote:
>
> Good afternoon Pros, this looks like rust. If so is there anything that can be done to clean this up. Looks like it through the stone completely in some parts.
>
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