I have used diesel on a 90 C2500 that was a construction truck, it had tar mostly below the belt line & the fenderwells were caked. I put it in a garden sprayer, soaked & scraped the thickest stuff off w/putty knife, then soaked again & power washed. It looked like new under all that tar. I did it in the driveway in front of my shop, as grass does not like diesel. Brian
