i just finished cutting the webbing out of the underside of my factory trunk lid on my '73 chevelle to shave weight.i noticed the underside of the lid is coated with a weird tobacco stain/ rust-colored substance which i think is an anti-rust coating from the factory.upon removing my interior headliner i noticed the same thing.should i sand this stuff to the bare metal before priming and painting or just prime and paint right over it? thanks for the help!