Filling cavities with stiffening foam vs. bodywork

I'm getting my 66 Bonneville back very soon and I'm totally stoked. Part of the plan involves reducing some NVH (which led me to that filling body cavities with foam thread).

This project has already been an exercise in diminishing returns, so why not go all-in?

My main concern is that if I fill all the little nooks and crannies with expanding foam then I won't have access to the back side of dings and dents. Is that a big deal? Body-workers: is it safe to assume that most of that can be done with a slide hammer from the outside just as easily? I don't want to go through all the effort if it will just have to be painfully removed everytime a door gets opened into my C-pillar.

I would be concerned with foam trapping moisture in the cavities and causing rust down the road. Other than that, I don't have much to add.

My gripe with foam products is that they hold moisture - so I would take a pass on filling inner panels - and would use a sound dampening sheet or two on big panels that might be a source of drumming and cover the interior with some of the newer sound deadening materials (like dynamat, only the much cheaper/better versions). YMMV

+1 on the moisture issue. In the 70s VW used a similar foam substance as insulation in the c-pillars of VW bugs. Guess where they all like to rot out in their later life?

I usually go to the dentist for cavities.

How about spraying the foam into a plastic bag in the cavity? It would have to be a heavy wall bag that could be sealed. Something like smaller bags made from the contractor/construction debris cleanup bags. Or heavy plastic drop cloth.

My Daytona came with pillows of insulation in the body cavities. The pillow skins were probably similar to a regular trash bag in thickness. They were stuffed with what looked like shredded jute padding. Unfortunately some mice got into it when the car was in winter storage.

Something like that could work for the larger cavities. Wouldn't worry about the small ones.

Yeah... the moisture issue crossed my mind. I have been assured by a few billion people that the foam is open-cell inside and closed-cell on the outside and won't wick moisture...

but after pulling about 200 lbs of "closed cell" poly foam out of my boat that was like cutting into a juicy apple, I don't buy it either.

I want to stiffen/deaden as much as I can. I have tried dynamat (or their alternatives) in four different cars with incredibly disappointing results. I couldn't tell a single bit of difference. I probably added 150 lbs to my s/w with RamMat and couldn't tell anything. It was still just as noisy as it was before I spent $300 on foil-lined-butyl.

I'm taking a different route with my van right now (but that is just for sound, not for rigidity) by using egg-crate foam covered with pegboard/carpet. Just the fact that those materials are sitting in the cargo area of the van has dropped dB significantly. I can't wait to actually install it.

The large cavities don't concern me as much. Things like the trunk/rear seat bulkhead I can reinforce by welding in an X-brace with 1" square tubing. I was thinking of filling the smaller cavities with the foam.

Ok... so we've determined that I need to do more research on the moisture aspect... how about the body work aspect?

body work isn't so much of an issue with stud welders. Might be an issue with paintless dent stuff.

Beware that some of the expanding foam products expand with some force. I bulged a less-than-foot square 1/4" plywood sheet pretty badly on a canoe floatation chamber project with some of that stuff. They make versions that don't expand nearly as much, specifically for doors and windows, that might be more suitable.

I have some spray-foam filling a cavity in the MR2 that is exposed to the elements on the bottom. It gets all of the road spray from the rear wheel. (temporary fill that has lasted a year due to my laziness in getting the patch welded in...)

It DOES NOT wick moisture.

I'd recommend going with Dynamat or one of the similar products on the market. Also, the spray on Lizardskin stuff looks interesting.

However, the first question is just how noisy is it in that car? The '65 GP I used to own was very quiet - there's a lot of carpet padding and upholstery in those cars to soak up sounds.

EastCoastMojo wrote: I usually go to the dentist for cavities.

Best answer!

The problem with foam often isn't that the foam itself traps moisture, but that you can get moisture wicked in between the foam and the metal if you don't have a 100% perfect application.

Alfa also used foam in cavities back in the 70s and on the Alfasuds, it worked as a rust accelerator (not that they needed one) because of this.

I used some expanding foam from a can to fill and stiffen the hood on my old van. 3 years later I had to replace it as the metal rusted through from the back side since the foam trapped water.

jimbbski wrote: I used some expanding foam from a can to fill and stiffen the hood on my old van. 3 years later I had to replace it as the metal rusted through from the back side since the foam trapped water.

True, but that stuff isn't designed for stiffening, nor is it closed-cell foam.

The real stuff (a two-part urethane foam) is supposedly closed-cell and won't wick water... but try telling that to me after I used the proper stuff to do my boat floor and two years later found my boat sitting 2" lower and had no tongue weight anymore. I pulled out hundreds of pounds of foam that was just like the pulp of an apple. I'd hit it with a shovel and water splashed out.

Didn't Ford advertise something like this in the F150 some years ago? I'd guess there is a specific type of foam you can use for this but it may not be easy to get nor apply.

pinchvalve wrote: It DOES NOT wick moisture.

That's not correct.

Though residential grade expanding foam such as Great Stuff is closed cell (sort of), it is still hygroscopic.

It's outer layer is a closed cell layer, which prevents it from wicking in the typical sense (ie, it won't sponge up water), but it CAN draw moisture from the atmosphere.

The temperature/ vapor differential on the 2 sides of the foam material encourage moisture to move from the warm (moist) side toward the cool (dry) side. In a car, this can be in EITHER direction. That's why sometimes your windows fog on the inside, and sometimes on the outside.

When the moisture moves toward the back of the insulation, no problem. It is generally an open area that allows it to evaporate. When the moisture moves toward the metal body panel, watch out. The moisture will hit the cold metal surface, causing it to condensate, and the insulation will prevent it from evaporating, holding it against the metal body panel indefinitely. It will rust the body panel from the inside out.

It will get worse when the insulation is a little older and the outer shell layer of the foam is degraded or compromised.

I know, because I have a car with thoroughly rusted rockers for this exact reason.

The GRM solution??

http://www.hotrodders.com/forum/alternative-lizard-skin-103610.html

It's cheap, done with readily available materials, and hasn't been experimentally verified. At the worst, though, you've painted the inside of your panels for $35. I'd give that a shot first, if it were me, before dropping several hundred on lizard skin.

I like that idea.