Ya I would imagine a good pressure wash or a few would be in order. From what I understand of the product the surface needs to be quite clean for it to adhere. Robin do you plan to do the whole underside or just the rear?
I would not have enough room to apply it it from the rear tires forward. It would get everywhere it shouldn't.
I also would want to have something to protect it from the elements. But I totally think that spray foam then line-x over the spray foam would be bomb.
I do wonder just how much line-x or something similar would help with noise and insulation just by itself.
Got a time lapse of the ceiling install.