Some of the panels in any car are going to have ripples and uneven surfaces due to the metal stamping. These ripples are what gives the panel its strength, not unlike how a rippled fiberglass surface is stronger than a perfectly flat one.
Sometimes these ripples and stamp patterns make it difficult to get sheet-based deadening products to adhere properly without leaving air bubbles and other imperfections that must be rolled out or even cut to release air.
Would there be any benefit to filling low-lying and rippled areas of metal panels with clay, in order to produce a perfectly flat surface on which to adhere deadening products? I can see it being time-consuming, but adding mass to thin metal can't hurt a bit, other than adding weight to the car.