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Old 07-05-2009, 09:59 AM
Brian Martin,Freelance adviser
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"Basics of Basics" Moisture shields behind trim panels.

“Basics of Basics” Moisture Shields
By Brian Martin

Moisture shields, those paper or plastic things behind door and quarter interior trim panels, are they really needed? I have removed a fair share of these interior trim panels to find no moisture shield there, left out during some previous repair. The car seems fine without it, are they that big of a deal? In some cars, they don’t look like they are doing much at all.

A BMW came back to the shop the other day and prompted me to write this “basics”. This BMW had a water leak on both sides of the car; the carpet was soaked. We had just completed a repair where both rear doors and one front door had leaks (wellllllll, maybe we didn’t COMPLETE the repairs after all.) The leaks looked like they were coming from the door seals themselves, possibly from wear. But after a closer inspection, water could be seen pouring out the bottoms of the interior door panels!

I removed the door panels to find that the “tech” (I use that name loosely when I see crap like this) didn’t properly re-install the moisture shields. This come-back would have never happened had they spent a minute and a half correctly re-installing the shields while putting the doors back together. What a shame, looking like morons over a minute of time “saved” during the repair.

Here is that BMW moisture shield, not stuck to the door at all. Water was POURING down the back of this shield into the interior of the car!

There are lots of methods used in the manufacturing of late model cars that should be used in updating your older car. In my opinion, this is one of those things.

We know that the moisture shield on a 1968 Camaro isn’t doing the same exact thing as one on a 2006 Pontiac Gran Prix. But why not have as good a water/moisture barrier on your Camaro?

On a 1968 Camaro the shield was little more than a “condensation” shield. And it was only really needed if the drains on the bottom of the door were plugged, holding in an inch of water. Other than that, it barely did a thing. While on that 2006 Gran Prix, holy cow, the “moisture shield” is literally holding back water, and LOTS of it from entering the interior! I was blown away after putting one of these cars together and giving it a water test before I had installed that moisture shield. The water POURED in to the carpet off the bottom of the window channel. I thought something must be wrong with the “seals” on the channel. I pulled apart the other door and had water ran over the window while looking from the inside. The water was POURING onto that moisture shield!

Today’s moisture shields are doing much more than the old tar paper ones from years past. Many of the cars today have plastic shields attached with butyl tape sealed like windows!

A tip when working with these butyl tape shields: If you try to just pull it off you will be tearing it all to heck. Pull out on the shield exposing the butyl tape and slice into it with a razor blade. Keep a constant pulling pressure on it while you slice a little into the butyl as much as the blade will go without using much pressure. Use a long, shallow slice. While pulling on the shield, make more and more slices until you go through.

On some cars like Hondas they are using a horrible, super sticky messy crap to seal their moisture shields. The stuff is just terrible to work with. But if you use the same method with the razor blade, you can slice through the stuff without making strings of this sticky goo to deal with. Then after you have removed the shield run a strip of 2” masking tape over it, so you can work on the car without getting this crap all over you. I am not kidding, it is the stickiest, most disgusting stuff I have ever worked with.

Here’s a 2010 Toyota Sienna. You can see the butyl tape as well as the hole in the door shell at the bottom of the moisture shield. This allows any moisture that builds up on the shield to drain back into the door.

This is one misconception held by many, that the door doesn’t normally get water inside. However, it does. There is no way you are going to have a moving window and keep all the water out of that door. The fact of the matter is, the doors and the quarters (if they have roll up windows) are designed to allow water to flow through them. The trick is, the management of the water! The drain holes in the bottom of the door have to be clear. The drain hole at the bottom of the moisture shield has to be clear as well, and the moisture shield has to be sealed properly. If all these points are operating, the water simply goes through the door and out the bottom just as it was designed. This is why you need to be sure your pinch weld at the bottom of the door is properly protected from corrosion, but that’s another “Basics”.

I had no plans on making moisture shields for my Gran Sport today but thought some photos of the process should be included in this “Basics”. I quickly did one so this may not look as nice as you could do for your car. However, how nice does it have to be, as long as it works.

Here’s what I used: Some 6mil plastic sheeting available at your local hardware store, roughly cut to size and taped up on the door to hold it in place while it is more precisely cut.

The original tar paper shields used a “strip caulk” style sealer and I think on these old cars that is plenty good enough. However, the strip caulk does break down, and I have pulled door panels off to find the moisture seal hanging like a flag because the strip caulk dried up and lost adhesion to the shield or door. Using butyl tape would be much better.

The strip caulk or butyl is simply laid out on the door on the outsides of any holes. The trim panel’s holes get filled up, so really they don’t need to be covered. However, with some older cars that use the spring clip style panel retainer, they may have gone on the outside of the holes with the moisture shield.

To use the Butyl tape, first buy the smallest one they sell, ¼”. But you also can stretch it out thinning it as well. If you were to leave it too thick there is a good chance the panel won’t click on properly. But if you stick the end down you can pull it, stretching it thinner as you stick it down around the door.

At the bottom of the door there is a slot to tuck the shield into. There are many different ways they will do this but all with the same idea: to tuck the shield into the door so any moisture will run down it (and back into the door) and out the drain holes in the bottom. My Buick has a slot the entire length of the door.

Tucking the shield into the slot…

Tucked in…

You cut a little “X” in it to push the window regulator shaft or power window wires to go through. The holes for screws can simply be punched with an awl before you put the trim panel on.

The butyl tape or strip caulk is ran up to the slot. Then after the shield is pressed down squishing the butyl tape or strip caulk, you run a strip of tape over the whole thing. The tape can’t be seen very well but the bottom flap is taped down in this photo. I used 1.5” packing tape. You simply run it over so half is on the door and half is up on the shield. You can barely see it at the end to the left of the white line on the right side of the photo.

There you go, now go install some moisture shields!