Dee and anyone else that is trying to use Dynamat/GTMat/Fatmat/etc...
This stuff is designed to dampen panel resonance. It's NOT an effective non-structure-borne sound absorption or dampening material. What it is good at is adding mass to a panel to change the resonant frequency and cut back on panel noise (structure-borne.)
To do this effectively, you only need to cover 25% of a panel. You don't need to cover the entire panel as that's a complete waste.
If you want to block sound, you need closed cell foam and mass loaded vinyl. You place a layer of CCF on top of the panel to act as a decoupler for the layer of MLV you add on top. THAT is what blocks sound effectively.
This is a great resource for effective and efficient sound deadening:
http://www.sounddeadenershowdown.com/cgi-bin/index.cgi