Try Sonus Leather cleaner...the key is multiple applications. Apply product to surface and let sit for 30 seconds. Scrub with firm foam applicator, then a toothbrush. The dirt should be removed when you clean the area with a clean microfiber cloth. Repeat process multiple times. If you don't care about destroying the surface, at this point try a solvent like Goo gone. Maybe this will work, or maybe it will destroy the surface. Nothing's wrong with trying though. Good luck.