I'm in the process of sand blasting the roof on my 69 firebird project. The car had a vinyl roof on it, so the roof metal has some pitting, as well as a little bit below the rear window. I'm using a black sand product from Menards that seems to work well. My question is: How can I tell if all of the rust has been blasted out; could I apply a rust dissolver to the roof after blasting to be sure, what is the black stuff in some of the pits after blasting? I will be top coating with SPI epoxy primer.