Does the surface that you stick the product to have to be perfectly clean? Reason I ask is I've got the headliner out of my Gremlin to change to a better one but I want to start adding some insulation. The inside of my roof has some surface rust and some left-over glue residue from the old headliner. Short of turning the car upside down, I can't really think of any way to REALLY clean the roof surface.