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What are you going to use? Fiberglass doors are already pretty resistant to resonance as is and IMO do not require the typical 2+ layers of deadener. Hmm I used about 1/2-2/3 gallon of edead v3 liquid deadener + 30sq ft of Dynamat Extreme + Cowboys kit. I am pretty resonant/rattle free. I used extra Cowboys kit to help tighten up some areas where I had rattles and it has worked well.
Liquid is only on/in the doors and in the front tub area. I have Cowboys kit throughout and I have Dynamat Extreme on the door panels around the speaker mounts and underneath and around the sub.
I used 3 bulk boxes of Dynamat Extreme. I found somes deals on Ebay that saved a BUNCH of money from regular stereo shops, well over half. I did all the hatch, drivers compartment, and the doors. It is alot quieter than before. Huge difference. You will be glad you finally did it!
On Ebay go to: Consumer Electronics > Car Electonics > Installation Products > Acoustic Dampening Materials
Watch the freight charges, some charge way more than others.
If I remember right, I think there are 9 sheets in the bulk pack. I just went and looked on Ebay and the prices vary from $85 to $120 plus the freight. Look at my previous post and it will show you how to get there.