One product that will not work on the exterior for sound deadening, is
Kool seal. I see this mentioned in the archives, and I can give a review
of this product.Kool seal is brushed on the wheel wells for sound
deadening.it one hold up.It has cracked and is peeling very badly.This is
after less than two years. However, a genaric product from Home depot ,
for roofing is holding up just fine(front wheels).Be concerned of anything
used on the interior. Vapors may cause a problem.

I used a under hood sound blanket from a late Buick for the rear hatch.It
is light and very good($5).The hatch will still raise up with the pistons.
Why not do the 4 walls with this?