you'll need some pretty good degreaser to soften that old crusty stuff before you can pressure wash it off there! Then, if you are doing it at home, you will have all that junk on your yard! LOL.
I normally just scrape/clean where I am working and call it good. All that caked on grease and dirt add a layer of protection to the underneath! Although it may add a bunch of extra weight and ruin gas mileage!
