Sheeot....I wanna see your pressure washer....either it's a badass or your undercoating SUCKED. I had to melt mine with a torch and scrape it off with a scraper. My 3K PSI pressure washer didn't touch it.
I can't remember the psi on it off the top of my head but I'll post it when I get home. This all factory 42 year old undercoating. You do have to get pretty close with the tip to do it.