Hi, I had trouble trying to clean some tight spot of a Merc E200 today. Used TFR, Ag tar remover and Ag paint renovator with a Mf cloth wrip around my finger nails and dig it. Only paint renovator is able to get ride of the dirt but i got to rub it so hard till my fingernails get pretty hurts. I am wondering why the stain is so hard to remove and is there any tips and easier way to dig out the stain? Some pics of the stain.