Crud on Side Marker inside Fender Well

Not sure it is undercoating. It seems like dirt that is compressed, big time. I "think" it is road grime from 45 years of driving. I will double check to see if it might be undercoating, but the car was not really worked on much before I got it. It sat for a number of years, but it seems like the must have driven it through a hundred mud puddles or something, lol?

If it is dirt how could I get it off. I can't get to the nuts to turn them, they are buried in this stuff. ugh But the dirt is like rock or something now, very, very, vvvvvvvery hard.