I have been working on removing the old under coating from the bottom of my 66 Lemans. I used a torch and a wire wheel to remove the old coating. The old coating was beyond saving and in the process I found small rust holes in the floor pans and trunk pans that I welded up. The problem that I am having is the black film left on the metal from the under coating. I need to remove the film before I spray Zero Rust. What is the best way to remove this film. I tried a little sand blasting but it did not work very well.