I use a ten-power loupe from Lee Valley to inspect those pieces of dirt under the top layer of white and off-white mats. Most times I can dig them out and restore the surface with a Wizard blade. The Bainbridges are the easiest to recover, and the Crescents are the hardest. There have been times when I just leave them, and no customer has remarked on them, but generally I take them out. Not a big deal; dig them out, dab the area with a clean wet cloth, come back later and work the area if necessary, but never when wet.