I actually sold all my mustangs to buy some property and that fell through . I used to double the factory type sound deadner . I use the caulking seam sealer ,if needed it can be spread with a small brush . I recently did a fast back conversion that had the sticky back sound deadener and couldnt get the stuff off ,wound up cutting 1 inch strips and scraped with a putty knife ,took almost a week just to remove enough of it to do my replacement panels ,i will never use it .