Thank God I'm here....

I would strongly recommend that you use a butyl rubber mat rather than expanding foam.

While foam will muffle sound waves within a void space, it lacks the concentrated mass to change the resonant frequency of a given panel. Also, it is incredibly difficult to clean up and since it relies on air to dry, there's a damn good chance you'll wind up with a gummy mess.

With my Integra coupe, I used Dynamat Extreme to completely cover the interior side of the rear quarter panels from the window sill to the floorboard, as well as the inside face of the door skins.

On the floorboards and the rear bulkhead, I used Cascade Audio Engineering VB2 mat over the OEM asphalt mats. Unlike Dynamat, CAE's VB2 is an asphalt-based mat with less mass per square foot.

The VB2 works reasonably well when pared with the OEM sound deadening mats, but I would not recommend it over Dynamat Extreme if you are planning to use it on your fenders or door skins. When / if I get around to doing the firewall, I may replace it.

So there you have it... :crazyr: