The Nattys Blue will leave the tell tail white marks on porous back trim. I was doing a quick 2nd layer on my hood, and did not put paper under the hood and close it to protect the wiper cowl. That's right white marks. Took some PB APC @ 15:1 and a grout brush and cleaned it off.
Take Gus-Gus's advice, if you don't want to do this, tape it off.