By the way - tannic acid from wood and plant fibers will also develop film. Has anyone tried leaching the barkdust in their yard yet to see how that compares with the coffee? You get a nice stain very similar to pyrocatechol with tannic acid.
All the bark, wood chips, etc. belong to my horticulturist wife.
But she's ok with me using some of our Rosemary (loaded with caffeic Acid, BTW).