I have had good success with removing coffee spots with a product similar to Hydrox that is based on Hydrogen Peroxide.
Best results were when I did the entire mat.
BE SURE TO ADVISE THE CUSTOMER THAT DUE TO IT'S PROPERTIES, SOME LIGHT FADING MAY RESULT.
This is better than having spots all over it, and most customers accept this.
Never had a customer that bought a second Sisal mat after having an "accident".
When liquids gets onto Sisal, it darkens, the H2O2 can lighten this back again, but it can also go too light.
I had one job that had water damage, where I decided to wet the entire mat out, then treat with the H2O2.
Customer was very happy.
Keep the % light.
Practice on Sisal mats from charity stores, etc;
I don't regret my past, I just regret the times I've wasted with the wrong people.
Take me as I am, or watch me as I go.
I'll retire when I can no longer do what I love, or I no longer love what I can do.
Stop moving, start dying........