Do you mean on the backside of the plastic D/P.'s? I would't go too crazy with that. A light coat would be O.K. Make sure that all the door panel clips are there & in good shape. The undercoating will add weight which may not seem to be too much, but through the vibrations of just driving & the bumps of manhole covers, potholes, etc, & that bit of weight magnifies itself pretty good. You'll need all the clips to hold the heavier panel nice & tight up against the door itself. I found that if my door panels, with no coating, are missing even just 2 clips, the door panels will rattle & shake over some of the bigger bumps.
On the inside of the outer door skin, I like to install deadener the same way as the way I did this headliner. 3 strips first. These ones are 6" wide. Inside the door I'd go 4". I run them across the longest distance. Like putting your hand across a guitar's strings to silence them.
Three is the minimum amount of strips. You could do five, or seven. But that's overkill. I only get up to those numbers on something like a '65/'66 Impala's headliner. (They're huge!) The reason for the odd number of strips is that this cancels out any vibrations/reverberations/resonances. Whatever you want to call'em. The first one is the center one. In the center of whatever panel your doing.. The others are centered from the outside edge of the first strip, to the outside of the panel.
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Then those strips get covered as completely as possible with more deadener.
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Once the rest of the metal inner door panel is installed, I'll cover that with deadener just to inside of the mounting-pin holes. Over the bolts & all. Even the vents at the bottom of the inner door. Once that's on, I'll go back with an exacto knife & cut all those out. You'll have to cut around the metal plate that the armrest mounts to. You'll end up with a hole above that. Cover it up with some more deadener. Can't find a pic of that right now.
If you ever have to remove that inner metal panel, you will have cut the deadener around the panel to get it off.
65-70% of road noise comes through the doors. That's why I spend so much time on them. Your door pins have to be good shape to handle this extra weight.
Here's the back wall in my blue truck. And a lesson learned. Don't try to do it in one piece. I tried. Check the wrinkles! 1/4 sections at a time works way better! Start from one side to the other & don't try to stick it in position all at once. Get about the first inch of one side stuck on, then while holding the other end away from the panel, work the deadener into the shapes of the wall from one side to the other. Overlapping the deadener is good. Not much, 1/2-1" works. I think that that adds to the integrity of the deadener.
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Yea, I did this with the seat in! Jack too. When I do the floor I'll fill in any spots I missed.
I then covered the back wall with 1/2" closed cell. This stuff's pretty thick. This is the only place that I would use it.
The foam has a a 'skin' on one side. This is the side that gets glued. I break the skin with something like 60 grit sandpaper. This time I used a wire brush. Side to side, corner to corner. This gives the glue something to grip.
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I buy industrial glue by the 5gal. pail & spray it through a paint gun. Too much for you, obviously! Do not use an aerosol glue for this. It will not hold. Use '3M' Automotive Trim Adhesive. Comes in a black tube. Probably need 3 tubes to do the back wall. For some rason, the black is more expebsive than the yellow. It's the smae damn stuff!
Spread it out thinly with a putty knife on both the back wall & the foam. Let it dry to touch. Follow the same procedure as the deadener, but dont over lap it.
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I found on mine that I when took the foam just up to the bottom side of the holes where the pins that hold the carpet go, the foam was so thick that the carpet pulled off of the pins. Cut some of the foam away, or just take it up to where the wall changes direction near the top.
I also did the 'A' pillars, when I did the H/L.
Some tips.
Use a roller on the deadener. Your fingers are not good enough to get it to completely adhere. Or to get it into all the weird shapes of the metal.
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If you get a spot that's going to wrinkle towards the outsides, push it down in the center moving to the outer edge. You'll end up with 2 wrinkles. Do the same for those. This help eliminate any really big wrinkles that may cause fitment issues.
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For big wrinkles, like in the footwells where there can be severe direction changes, cut straight into the middle of the wrinkle, and overlap the cuts.
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I've been trying to find 1/8" closed cell foam. I bought about $500.00 worth about 2 years ago off of ebay, but that seller is no longer a member. I'm gonna need some more, myself, soon.
I like the 1/8" better than the 1/4". As that stuff gets thicker it doesn't like to conform to tighter shapes. Two layers of 1/8" fits a lot better than one 1/4" layer.
