For the doors, I'm going to go with the 25% coverage idea //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif - as part of a complete treatment. Here's what I do: Start with 25% coverage with a vibration damper on the outer skin. I also stuff some of the Extruded Butyl Rope I sell between the outer skin and the side impact protection beam. This basically uses those parts of the car to make its own vibration damper. Then I put a little bit on the inner door skin. This is enough to control resonance in the sheet metal and make the door shut with a nice solid sound thunk, but it isn't enough.
There are two basic issues with doors. First, they are very transparent to noise, particularly traffic noise. Second, they make terrible speaker enclosures. I'm a big advocate of building removable covers for the access holes, but whether or not you choose to do this, the next steps make all the difference in the world.
I hang a layer of mass loaded vinyl (MLV) on the inner skin using Velcro Patches to help while I'm fitting it and to make it easily removable for future maintenance. You want the MLV layer to be as large as it can be and still fit inside the trim panel when it is replaced. You'll need to cut some holes in the MLV to allow rods, shafts, cables, wires, speakers, etc. to pass through. You want these holes to be as small as possible. Take special care with the mounting clips. You'll need to cut holes that are large enough to let the panel clips fully seat. It's often easiest to do this by laying the MLV inside the trim panel after you have done the trimming you need to do on the door.
The MLV does a few things. It's very effective at blocking outside noise. It also reinforces the baffle plane - isolating the front and back waves coming off the door mounted speakers. It also replaces the vapor barrier.
Finally, glue a layer of closed Cell foam (CCF) to the MLV on the side facing the vehicle's interior. Now when you put the trim panel back on the CCF will compress slightly applying pressure to it and the sheet metal. This helps control rattles and buzzes in the door.
Here's a thread that illustrates the process. You'll obviously need more material for a Vic than the Pontiac Solstice, but the idea is exactly the same. I've done many cars using this technique and my customers have done many more. I've also done many cars using the old multiple layers of traditional sound deadener everywhere approach. There is absolutely no comparison.
