Underbody Rust Protection

[Manche757]

I am planning to have body work and painting done on an 87MJ in May and want to have any underbody coating/painting done before that.  A discussion thread of what some of you have done and the results would be helpful to us all.  After all mechanical work has been done, what have you done to seal up things from the weather and winter hazards?

[Pete M]

Our trucks rot from the inside-out.  whatever it is that you choose to do has to be done inside the frame, inside the rockers,  inside the bed crossbars, and up inside the bedsides above the wheelwells.    and be sure that anything you apply isn't trapping the salt in a new place.  salt and moisture will get everywhere.   It's best to just plan for that rather than trying to keep it out. 

 

Honestly, the rest of the salt spray can be washed off in a car wash.    it's the places you can't wash that are the real trouble spots. 

[Chad R]

In the fall 50/50 diesel fuel and motor oil in a bug sprayer. Spray inside doors, inner fender wells, inside tailgate inside rockers and everywhere you can think water can get on the under side. Let it sit and drip for a few hours. Then a day or two later wash the out side of the truck. The truck will not smell like diesel. We do it to my dads 07 dodge 2500 4x4 no rust and yes I am in Michigan. Or you could do fluid film and do the same thing. But you are going to have to do it every fall

[Minuit]

Ditto what Pete said. The enemy lies on the inside, not necessarily the outside. Here's how I'd go about it. In fact, this is exactly what I will do on my 89 when it comes time to re-floor it and do an actual good job on rust removal.

 

The only type of rust preventative products that I can 100% vouch for are oil-based and wax-based coatings. Krown and Fluid Film are two well known brands. DO NOT under any circumstances use a rubberized undercoating like the factory material. That was applied to a clean, brand new frame at the factory. Otherwise, that stuff will do nothing but seal in moisture, dirt, and rust. By the time you see rust on a Ziebarted vehicle, it is much too late.

 

Fluid Film and Krown are transparent - if there is rust underneath them, you can see it. They do not tend to seal in rust, and if applied over rust they tend to prevent it from spreading.

 

Just spraying or brushing stuff on top of rust is not a solution. The entire inside of the vehicle needs to be looked at. If original paint is present on the floorboards, that can be used as a map for where to focus your efforts. If it's been painted or POR-15'd over in the past, the whole floor needs to be wire wheeled back to bare metal to find the rust. This means looking on the inside, not just at what can be seen at the surface level, and yes, I mean looking inside of the uniframe too.  A big hotbed for rust is the angled brace just inside of the door openings. This piece must be removed and the area underneath checked for any signs of rust. Any rust needs to be dealt with and that panel needs to be welded back on. After that, fluid film on the inside of it. Anything less is a half measure. You're fighting a war against rust, and it must be stamped out at its source and prevented from returning at any cost.

 

A borescope camera is a big help in looking inside of the frame without cutting it open. They make ones small enough where all you need is a 1/2" hole to look inside.

 

 When you have the work done, have them shoot Fluid Film, Krown, or an equivalent - NOT ZIEBART OR ANY OTHER "RUBBERIZED" PRODUCT - into the crevices in the rockers, cab corners, etc. It's not uncommon in the northern states to drill holes into the rockers and other hollow panels to get more coverage. As much as I hate drilling holes in MJs, this might be a path worth pursuing. The holes are typically plugged with removable rubber plugs, and every year the spray is reapplied to the inside of the body. They make special nozzles for spray guns that allow for full coverage to the inside of a hollow panel. This should be done after any rust is taken care of.

 

The interior must be waterproofed. That means everything. That means someone who knows where to look needs to be inside the truck with no interior installed looking for any possible water intrusion while someone else blasts the hell out of the truck with high pressure spray. The foam firewall gaskets are a big leak point. They must be cut from a suitable foam stock. The OEM ones aren't available anymore, don't even try. The hood seal cannot be relied on to provide a full seal, and thanks to the design of the firewall, water will drain right past several potential leak paths. To name a couple, the bulkhead connector below the brake booster, the antenna grommet (this is a big one and both of my MJs have had severe water leaks from there), and the foam gasket between the HVAC box and the firewall at the blower motor.

 

Naturally, a similar process should be followed on the underbody of the truck. Where paint is still intact, fluid film once a year. Any surface rust needs to be dealt with. Sand to bare metal, prime, and paint. Then fluid film once a year.

 

A related but overlooked thing is chassis lubrication. I'm not talking about grease fittings here. All hinges and points of metal-on-metal moving contact need to be lubricated to prevent rust from starting. I like white lithium grease for this. If you look at the door hinges, tailgate hinges, and similar on any of my vehicles, you'll see white grease on any point of contact. I try to do this every couple of oil changes.

 

It's not an easy job, but if you want your truck to last another 30 years without rusting, you have to do it the hard way. Wanna know why so many rust repairs last a couple of years before the truck looks like a rusty POS again? They don't do it the hard way.

 

I would not trust the garden variety mechanic with any of this. You need a guy with experience in rust repair and prevention that lasts. Familiarity with the MJ platform and its common rust points is a bonus.

[ghetdjc320]

12 hours ago, Chad R said:

In the fall 50/50 diesel fuel and motor oil in a bug sprayer. Spray inside doors, inner fender wells, inside tailgate inside rockers and everywhere you can think water can get on the under side. Let it sit and drip for a few hours. Then a day or two later wash the out side of the truck. The truck will not smell like diesel. We do it to my dads 07 dodge 2500 4x4 no rust and yes I am in Michigan. Or you could do fluid film and do the same thing. But you are going to have to do it every fall

 

This is an excellent and cost effective technique. Seen it used in many other countries to preserve the underside of vehicles. Works awesome! If you prefer a wax based product, pb blaster makes a corrosion stop that leaves a nice thick film behind that lasts a while. I am bombarded by salt spray where we live and use a car cover, wax, and spray salt-away frequently. But seriously, diesel works great.

[Minuit]

 

Here's a little follow up to my previous post - with our trucks and the way they are built, you have to look INSIDE as well. What you can't see is much more important than what you can see. Our trucks are made of a bunch of sheet metal spot welded together, and as a result there are a LOT of voids and overlapping joints in these multiple-layer sandwiches where rust can hide until it's much too late.

 

Situation: 1989 Jeep Comanche with long-ignored clutch master cylinder leak. Passenger side floor is in typical shape for a Tennessee truck; surface rust with slight pitting, but there's no clear signs that any sheet metal will need to be replaced. Driver side floor:

 

I know it was the CMC that caused this because there's a very clear trail of rust running up the firewall leading directly to it.

 

Now, I've seen a bunch of build threads on this site where damage of this type was addressed by cutting out anything with holes (usually the flat parts on either side of that floor brace that runs from front to rear), replacement sheet metal welded in, riveted in, or even sometimes glued in, and any other rust visible from this angle was ground down and painted over. That's a half measure at best, and in my humble opinion those people would have been better off doing nothing because in 5 years they're going to be really surprised when their truck rots through about 3 inches away from the repair. Here's why: the floor pan in our truck is not one layer of metal. In some sections, there are three or more layers of metal with voids in between each layer. The longitudinal frame brace that runs from just under the pedals all the way to the very back of the cab is the top layer, the layer of metal that forms the majority of the floor pan is still there underneath it, and finally you have the bottom of the uniframe which itself is sometimes multiple layers. In some areas, there is additional reinforcement, so there's even more layers than that. Here's my best 8:30 PM attempt at a diagram:

 

This is also my "it's not that simple in reality" answer to why it doesn't really matter if they make reproduction MJ floor patch panels or not - to get a correct repair, you'd need repros of at least the upper two individual panels, as any truck needing rust repair probably needs repair to at least the upper two layers.

 

Before I got stabby at the longitudinal brace with a screwdriver and poked right through, you could be forgiven for believing it was just surface rust with heavy pitting, and the correct solution would be to grind it down and POR-15 over it.

 

My argument is that the whole thing must be dissected and inspected from underneath, because sometimes there is even worse rust hiding underneath panels that at first sight look saveable.  To put some pictures to that argument, I'm going to cut out a slice of the longitudinal floor brace, which is completely beyond help anywhere past the rear seat mounting studs.

That's the main section of the floor pan, and there was no screwdriver action before this picture was taken. That's exactly what it looked like when I first cut it. Underneath you can see the inside of the uniframe, and just out of frame to the left is the lower control arm mount.

 

The real purpose of this expedition: to find out if the inside of the uniframe is solid. This is where a borescope camera would be very handy - just pop out a body plug and look inside.

 

The bottom of the main floor panel is visible at the top of this picture. Where I cut seems to be the worst of it, but you can tell the rest of it isn't in great shape. This picture was taken facing the rear of the truck.

There's lots of rust flakes and other crap in here, but otherwise looks fine. It wouldn't have been much longer before the rust spread to here, and some of the uniframe's flanges where the uniframe, main floor panel, and longitudinal brace come together are suspect as it is.

 

My point: the worst of this rust would have gone completely unseen had I not known that these trucks rust from the inside out. If you want to fix the issue for real, the sandwich needs to come apart. That means the spot welds holding the two frame braces need to come out, and the parts need to be inspected from the inside. If only surface rust exists on the bottom, it needs to be fixed and the assembly welded back together. When this happens, the inside of the uniframe needs to be inspected to make sure there isn't rust in there too. I'd recommend shooting a healthy coating of Fluid Film or similar anti-rust coating anywhere you can get it inside the uniframe.

 

Now, how do we fix this the right way? That's a good question, and I'm not fully sure of the answer myself. I really don't think replacing it all with a single flat sheet is a good idea. As janky as it seems to make a truck frame out of a bunch of stamped sheet metal squeezed together, the panels are laid out the way they are for a reason. The floor braces add stiffness to the whole floor section, which don't forget is a structural part of a unibody truck. Making exact or close reproductions of the individual parts from scratch and rebuilding the original assembly would be a ton of work, but until someone comes up with a better idea it's what I lean towards.

 

Once you've fixed it, how do we stop it from happening again? Two big things:

 

1 - WATERPROOF THE INTERIOR! Shame that Jeep didn't bother to do that from the factory, but that's just how things are. All gaskets need to be remade and the truck needs to be extensively water tested from all angles. Pack the hell out of the wiring harness bulkhead with waterproof dielectric grease, and make sure the clutch master cylinder if equipped does not leak. That's a really bad day if it leaks and is ignored.

 

2 - DON'T PUT A BUNCH OF SPONGES ON THE FLOOR! Jute padding and whatever the water-absorbent foam that lines the firewall are what I'm thinking of here. Your carpet underlayment needs to be something that doesn't hold water if leaks were to happen. 1/8" closed cell foam followed by 1lb/sqft mass loaded vinyl is a great choice here and will MASSIVELY improve the quality of the driving experience by reducing a ton of road noise. I do not recommend anyone put vibration dampers on the floor, as that's another water trap if the adhesive fails. Ask the driver side door skin on my '91.

 

Sorry this turned into a soapbox, but I'm tired of some of the half repairs I've seen, and I've been wanting to say this for a long time.

 

 

[Pete M]

and you're going to turn this into a full writeup, yes?

[PCO6]

On 4/2/2020 at 10:23 PM, Minuit said:

2 - DON'T PUT A BUNCH OF SPONGES ON THE FLOOR! Jute padding and whatever the water-absorbent foam that lines the firewall are what I'm thinking of here. Your carpet underlayment needs to be something that doesn't hold water if leaks were to happen. 1/8" closed cell foam followed by 1lb/sqft mass loaded vinyl is a great choice here and will MASSIVELY improve the quality of the driving experience by reducing a ton of road noise. I do not recommend anyone put vibration dampers on the floor, as that's another water trap if the adhesive fails. Ask the driver side door skin on my '91.

 

Great posts Minuit.   On this particular point, for those who use their Jeep (or?) year round in snow/salt conditions your choice of floor mats is important.   Last fall I replaced the driver's side floor pan and torque box in my LJ.   Floor pans are relatively easy but torque boxes are a much more involved.   The passenger's side and both rear floors were in great shape.   The previous owner of my LJ used simple flat rubber floor mats.   I replaced them with Husky liners but it was way too late.   For several years before I got it the driver's side footwell carpet never dried out in the winter and that's what caused the rust.  

 

Other than that, I have my vehicles sprayed with Krown T-40 annually and I spray them myself with Krown and a Shutz gun as a I work on them.  It's not a messy job and it's well worth it.   It makes working on them much easier too … everything is lubricated.

 

Your floor pan drawing and pics are very helpful.    I've done several floor and sill replacements (XJ's and MGB's mainly) and I have always applied POR-15 to the inside of the uni-body frame before I close them up and used seam sealer after that.   

 

[Manche757]

Those are some great in depth help