[Ender]

I've seen some of this type of crud on the surface before, you may not get it off of there with passive cleaning techniques like hot tanks and just being sprayed with penetrating fluid. You'll have to scrub at it. Don't use anything very abrasive though. I guess just try a clean cotton rag with acetone first and scrub hard, if that doesn't work you might try a really really fine sandpaper, but use it very lightly and ONLY on the stained areas. If you try sandpaper you might just sand a little bit then try the rag again to see if it just needed to be loosened by having its surface broken.