Full Size Jeep Network

This seems like an easy/obvious thing in retrospect; maybe someone will be interested.

Long ago I noticed that my J10 had these plastic plugs in the inner bed:


Shown here with some recent snow. Whoever did this placed the plugs at what seems like random intervals.

Clearly these were meant to inject some cavity wax or other rust preventative in the space between the inner walls and the outer steel. This area rusts badly on these trucks, resulting in the characteristic bed rust.

This is from someone else's post asking about this rust.



The J10 is long beyond saving in this area, but my J20 is relatively intact. Seemed like a good idea to punch some holes in the bed and inject ... something... to prevent rusting.

If you remove the cover over the filler hose, you can see what's going on here -
There are vents (seen here) but apparently condensation accumulates in the groove between the bed floor and the outer bed skin. There's no access to this area beyond this small section that's covered by the removable cover.

Just out of interest - my truck also has a rear filler that goes to an aux tank where the spare tire normally goes.

Years ago I bought a Fluid Film kit that included an Astro Pneumatic shutz gun. Works good at making an atomized blast of the supplied goop. First I sprayed the area behind the cover. Next I laid out 1/2" holes neatly distributed on the interior wall.
A step drill is excellent for this. I mark the step I want to drill to with a Sharpie pen; works well, but it wears off after a few holes.
Here I'm shooting the Fluid Film goop (lanolin) in. This is step 1 of refurbishing the bed with new bedliner (likely Duraback dark gray), since the bed liner the PO applied was poor to begin with, and has continued to weather. I also removed the worn-out bed liner that the PO painted on top of the bed rails (which looked awful IMO).
These are the caps that go over the holes. These are 1/2" CapPlugs, available on Amazon for about 20c each. I used the end of a hammer handle to push them in (my thumbs are not strong enough to do more than a few).

All finished.
I believe that extra hole in the bed floor would be used for the factory aux tank that was available for J20s.

Plugs: https://www.amazon.com/dp/B00PF0D0ZM

Shutz gun: https://www.amazon.com/Astro-4538-Econo ... f=pd_lpo_1

Fluid FIlm: https://www.amazon.com/Fluid-Film-Preve ... 10-1803025

This is an awesome write-up! I will have to do this on my truck as well!

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Thanks Tgreese, John is right. Awesome tech tips. Now I've got another job to do before this project I've taken on is ..."done"?

Thanks for the kudos guys. Not a plug for Fluid Film; just what I had on hand. Project Farm has a review of these products that you might watch: https://www.youtube.com/watch?v=lyWHF4NoNVk The Cosmoline-derived product looks good too, but maybe not available in bulk. Spray cans of cavity wax (3M, SEM or? Search) may be another option, esp. if you don't have air.

Just curious here, I know a lot of guys swear by the POR-15 undercoating (including me to some extent, although mostly because that's what I've used and it seems to work ok). What's the advantage of using this type of setup over say POR or another undercoating? Do the holes you have to drill not promote rust somehow?

POR-15 is a paint, not an undercoating. Works ok from what I've read. It's a MCU (moisture cured urethane) which chemically incorporates any moisture in the surface rust that is present. I think there are better MCUs on the market, but the POR-15 has a lot of popular exposure and is a recognized brand.

I suppose you could use POR-15 as a spray-on undercoating, but I don't think it would be very effective, and it would be wildly expensive.

Fluid film and similar products are intended to be sprayed on to protect surfaces from rusting. Cavity wax is meant to be sprayed into otherwise inaccessible places like rocker panels, frame channels and such to prevent rusting. Here I'm using Fluid Film as a cavity wax. This application will always be hidden in the bed sides, and never exposed to washing.

You understand there is a big empty cavity between the outer bed steel, the inner bed steel, and the bed floor in these trucks? The only access is the 1" by 1/4" vent that shows in the picture above. Maybe you should reread the above post.

If you are interested in characteristics of this type of product, here is a review video: https://www.youtube.com/watch?v=lyWHF4NoNVk&t=48s

The holes are capped effectively water-tight and the hole itself won't have any affect on subsequent rusting. The only new steel exposed is the hole edge that is coated in Fluid Film. The holes provide access to the hidden compartment so the cavity wax can be applied. Their entire purpose is to reduce rusting, not cause it.

I haven't spent much time on J trucks. I know what they look like from the outside and what they share with the Wags, but that's about it. I was assuming that like my dad's dentside Ford, it's pretty much just sheet metal on the outside, welded at the top, with a few braces on the bottom to keep it from flapping around. I'm pretty sure on those you can crawl underneath and spray up there (it'd be easier to take the bed off and spray up there, and you could do the frame, but it's doable while it's on), I figured the J trucks were similar. Sorry. Aren't all the pictures from up top? What's underneath to keep you from getting in there?

Also, POR-15 DOES make a roll-on undercoating, and that's what I've been using on my Wag. I think it's labeled as "chassis coat" or something like that, but it's the same diff. Yes it's expensive but if you keep it coated, it works.

Side note
Why on earth would Jeep have put that cavity there? With temp changes and stuff that's betting for moisture problems, and they know it's not getting painted in there. I can understand there being a gap between the inside of the bed and the bedsides, but why would they box it off from the bottom? As long as you don't have any shelves inside for stuff to get built up on w/o draining, it's not a problem.

sierrablue wrote: ↑Thu Mar 02, 2023 5:04 pm... What's underneath to keep you from getting in there? ...

The bed floor connects to the outside wall. Complete platform to the outside wall, sealed except for the small vents you see an example of in the above picture, and where the filler neck cover is.

Why on earth would Jeep have put that cavity there? ...

I'd guess it was an easy way to add an inner wall to the earlier single-wall bed. It's a problem with these trucks, no question.

Thanks for being patient with my lack of knowledge on this.

So...anybody redesigned these J10 beds?

That’s what I did with my bed kind of with the patch panels. It removed the seem between the upper and lower bedside, which is where the bed floor also meets, so it’s removing part of the issue. I’m pretty sure that is how the replacement bj’s offroad bedsides are as well.


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sierrablue wrote: ↑Thu Mar 02, 2023 7:25 pm Thanks for being patient with my lack of knowledge on this.

So...anybody redesigned these J10 beds?

Not sure what you could do. No profit potential, I expect. If you look under the truck, there's no access to this area. I presume much of the structural strength comes from the seam with the inner and outer skin. If you had the bed upside down maybe you could use a hole saw and drill some access from below. This might cause other problems from splash and such.

I would like to use a borescope and see if the interior space is actually bare steel. The part near the filler neck is painted.

Looking at this later, I added some labels that may be helpful -


The compartment between the inner and outer bed sides is open here where the filler neck goes. Normally this area is covered with a steel panel that's secured by screws, one seen in the picture. You can see the outline of where the panel fits on the bed floor, boxing in the filler neck. Elsewhere in the bed, the only access to the compartment between the bed sides (inner and outer wall) is the series of small vents roughly 1" x 1/4"; an example of one is seen in the picture.

My 76 bed has these all over.. theyre on my sill plates too and the doors. Figired this was factory