Undercarriage Rust Removal and Undercoating on 5th Gen Toyota 4Runner

First Attempt

Condition of the Undercarriage

Condition of the undercarriage upon purchase of the car left a lot to be desired. Skid plates and spare tire were simply scraps of metal that soon found their place in a trash bin. Bed floor had a large hole, all of the components were covered in rust, paint was peeling and bulging out in any location moisture could get through.









Fixing Bed Hole

Patching bed hole was a simple fix. I sanded the surface with rotary tool, and applied drywall sanding screen on top of it to provide reinforcement.



After covering the hole with Bondo Fiberglass Filler the problem was solved.



Initial Attempt at Undercarriage Rust Removal and Undercoating

Initial attempt at undercarriage rust removal and undercoating was a bit more complicated than the second one. I used all the tools available at my disposal to prepare the surface before applying rust converter. Wire brush, angle grinder with wire wheel, chisel, and screw driver got rid of most of the the rust and loose paint. The surface was then degreased and thoroughly cleaned, before application of several coats of FDC Rust Converter Ultra.



The surface turned purplish in color after the first application, and then eventually turned dark as this stuff dried.






Final Result

After the surface was fully dry, I applied 3-4 coats of Rust-Oleum automotive rubberized undercoating. The final result was very satisfying.





Assuming that these steps would be enough to protect the car from formation of new rust, I stopped here. 6 months later, after one Colorado winter, newly sprayed undercoating was showing signs of rust once again.

 

2nd Attempt

Initial attempt at rust removal and undercoating undercarriage didn’t prove to be a success. After spending winter time in Colorado, rust started to reappear above undercoating. Undercarriage of the car was slowly returning to the same condition as before. After some research I came across a forum post about Fluid Film where the same thing was mentioned.

The factory paint doesn’t last one winter, nor does an aftermarket paint product. Is it better than doing nothing? Heck yeah. And if you keep on top of it then it will work great. But it isn’t some spray it and forget it solution that people think it is. You would have to reapply it every year or two to prevent rust. The stuff they use on the roads here is just that awful. We have brand new vehicles for sale with rust on the frames because of test drives in the winter. Buying a 2023 in winter of 2022? It will have frame rust already.
https://wranglertjforum.com/threads/fluid-film-black-vs-fluid-film.65188/

This time, I spot cleaned everything I could reach with a my hands.





I ordered a gallon of black Fluid Film along with a cheap spray gun from Amazon. Since I was doing it in my driveway, I covered area under and around the car with a cheap tarp from Harbor Freight Tools. I also bought myself hazmat suit from Amazon for personal protection.

It is mentioned in many places on the web that Fluid Film needs to be heated to proper temperature before applying it. I decided to follow this advice.

I had a few doubts before applying this stuff. First, it seems like people usually apply it once a year, in late fall. Second, sand sticks to it. The timing just didn’t seem right. I was was planning a road trip that would take me through pretty rough terrain, which would be covered in dirt, gravel, etc. It was also the beginning of summer time.

I thought that some sort of protection is better than no protection at all, and went ahead with application. The whole process took me about an hour, not taking clean up into account. I ended up using entire gallon. Sections that were difficult to reach I covered using small brush.





After application of Fluid Film, I wiped off areas where important fasteners were, and applied Torque Tamper Detection Marker.

 

spraying everything with fluid film will do slow rust. spray FF inside frame more important, than paint or rubberized coating outside.

 

Afterthoughts

After application of Fluid Film I drove through dry and dusty areas of the American West. Most of the driving was done during the summer time. Except several water crossings in Utah and frequent rains storms in the North of the US, the undercarriage stayed mostly dry. All the dust that collected on the Fluid Film and hot summers have created a layer of dust that looks like rust because of the color of the dust in Utah.

I am not sure how much moisture protection I am getting at this point. Probably none. Underneath that layer of dust is a rusty a rusty undercarriage. New suspension components that were covered in Fluid Film have developed spots of rust all over them. Very strange. Not sure if Fluid Film and dust had anything to do with this.

I will have to redo the whole undercarriage the third time. I live in the Southwest of the US, but travel frequently to other areas of the US where moisture is a problem. This time I will definitely avoid using Fluid Film and will try POR15 instead.

 

I'd skip the POR-15. It's great for a muscle car restoration that never sees inclement weather and looks pretty. It ends up just like the old school undercoatings that end up chipping and you end up with water/salt collecting underneath.

 

Any suggestions on what to use instead? I have read about these negatives of POR-15, but at this point I am left with very little options.

 

The bed almost seems like it rusted from something that spilled inside. The plugs probably kept whatever it was from draining out.

The rest of the frame didn’t look too horrible. But I think it was a good call to do something anyway.

The cold hard truth is that there is no spray and forget solution to keep rust away on a vehicle. Most people don’t bother with corrosion control maintenance because most car will last you 15 years before the wheels rust off without doing anything.

But if you don’t want rust to limit the useful life of a 4runner, you have to spend time under it. The solution that made most sense to me was the wet-film products like fluid film and wool-wax. Yes, there will be areas that will need re-application every year or two or three. But not the whole car. I do a lot of my touch ups with a brush and bulk woolwax.

And yes, crap will stick to it, but some say it actually improves the coating’s durability. You still need to touch things up occasionally. Mine looks like chewed bubble gum kicked down the trail, but as long as I know there is no rust, I don’t care.






Below is an example of what needs yearly touch ups. Basically, anything that’s subject to abrasive erosion from road grit in rainwater or gravel. That means the front of the rear axle, rear shock mounts, the lowest end of the trailing links, and the bottoms of the LCAs and rear diff if you drag it over ground. I like to clean all the wool wax off to see what I’m dealing with (for example, to paint scratches or area where paint was sand blasted off) then I hose it down with wool wax (figuritively, since I like to glop it on with a brush).



In your case, you might have to keep an eye out to see how the rubberized coating reacts the fluid film. If it softens it, try to see what you can peel off. If it’s on there good, just do an inspection and a wet-film touch up every year. Make sure you spray fluid film inside of anything that has an inside!!! Frame rails, all 7 crossmembers, LCAs, body mounts, everything!! Till it drips out!

And even though a fully coated frame shouldn’t need to be underbody sprayed at the carwash, I still do it frequently in winter. It has minimal or no impact on a wool wax coating.

 

Agree on the warnings with por-15. To do it well, in a way that eliminates the problems of peeling and creating water pockets, really requires extremely good prep that’s probably best done with the frame on the rotisserie. The other option is doing all the steps of steel prep that the manufacturer recommends while on your back. Since you have already done rubberized and grease coatings, the amount of effort required to do it right is doubly prohibitive. And por does nothing for the inside cavities of the frame. I’m not really sure why it’s named POR, if they also recommend using an etcher product. POR would make the most sense on bright shiny clean metal that you can easily prep, otherwise it will only adhere as well as the base layer of paint it rests on.

 

I got the Krown undercoating applied a few months ago. I've also done a lot of research since then watching a lot of testing. It seems the Krown is okay and better than nothing but the lanolin based coating seem to do the best.

I live in Northern Michigan so I feel your pain but from what I've seen, the Fluid Film is the best or at the very least, shares the top position with a couple others. Everything I've seen indicates you want a good place to get it applied once a year. I also believe doing some yourself in between annual treatments is prudent especially if you are in harsh environments that can either soak up the product or when it gets washed away from a car wash or heavy and frequent mud/water crossings. I wish I had a better suggestion for you but it's the conclusion I've come to.

 

Thanks for examples and solid advice everyone. I already have all the things for Fluid Film application, so it shouldn't be a problem reapplying it once a year.

I applied FF over bare metal, so it is hard to see where I would need touch ups. It is probably better to just do the whole undercarriage. Would I need to power wash the entire undercarriage before applying a new layer, or I can just apply it over all that dirt and it will soak into the old layer and rejuvenate it?

 

As long as there aren't dried chunks of mud on the frame, I just spray over the old stuff. If it’s muddy, I just use the jet setting on my garden hose nozzle to clean it off (I just use plain water). It will still look dirty, but no big deal. Applying WW over previous layers builds up a nice dry crust that way as it gets saturated with dust, grit, etc. After one or two seasons you probably won’t see a need to recoat the whole frame. Just the touch up areas that need it, it will be easy to spot them because they wont look like nasty dirty woolwax, they’ll just look like your original rust.

I think in your situation with preexisting rust, I’d pay extra attention to the insides of the frame and crossmembers. Making sure salt mud and silt is cleaned out, and then saturating it with fluid film going to extend the life of your 4runner the most. I think if you do that once really well, you wont need to worry about the inside again.

By the way, i’ve read your other repair threads. Massive respect on doing all that work and posting the details. Great reference material!

 

I had some simple surface rust on my tundra, sprayed new Hampshire oil on it. To the eye, looked like a win. 4 years later I don't see the same state it was in. Not black, but no light rust I saw before. Sorry, no pics. Not sure if you've looked at it or not, but I used NH oil back to black.

 

That sounds interesting. I will look into that. Seems like it is specifically designed for undercoating.

 

How about Eastwood Rust Encapsulator? Applied to rusty areas after using wire brush on ....have heard good things with this stuff ....thoughts?

 

I am very tempted to try used motor oil mixed with diesel in 50/50 mix. Seems to be old-school way of accomplishing the same thing. Going to stop by AutoZone and get some of their used oil today.

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Video 1

Video 2

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Some useful tips I came across in comments:

- the drain holes, I saw a guy tape them and drive the car around a while then untape the and let them drain
- drive down the dustiest road you can find after applying to the oil to stick the oil to the car
- i use diesel and used motor oil 50/50 in a craftsman airsprayer. works great!

 

no don’t do that motor oil will dry up and fuck up your seals use woolwax or fluid film if you’re still getting rust you’re doing it wrong

 

Yes, after doing a bit more research this is exactly what I found. Motor oil is not so good for rubber. Good call on that. I wounder why it worked for the past generations so well.

 

COSMOLINE. Stuff goes on like a paint and dries to a very tough wax. Will not come off unless you want it off using wd40, gas, diesel etc

 

I wonder if cosmoline might work better on pre rusted metal than Wool Wax would. Just because it sets up waxy. Seems it would be a tougher barrier against air and water.

@strannik might be worth getting a can to spot test on top of the rubberized coating you have on now. It might actually loosen it the rubber. If it does, that means you’d have to strip it before coating (more work,yuk).

 

I don't think the problem with the first attempt was the paint, I think it was the metal prep. I've found rust converter to be no better than metal primer over small patches of rust, while the primer does significantly better on clean metal than rust converter. If you had used a normal primer, I think it would have gone better.

From my experience, black enamel rustoleum paint is a good choice. It's not explicitly designed for frames, but with some metal primer it lasts. But removing rust is the most important step, my success using it on my frame is dependent on how well I do the prep. I also used it on my roof rack, and it's been great there too.

I've tried CRC Marine Corrosion Inhibitor, it dries to candle wax consistency. You can scrape it off with a finger nail, but it isn't sticky at all. It's been very effective, but due to high cost I only sprayed it inside my frame. Fluid film is good, but dirty, and it won't stop rust that's already happening. I use it on top of my undercoat paint, and the combo works.

 

3rd Attempt



I think I know what happened. When I was posting this image two years ago, one of the posters mentioned that this is not rust. To him, it looked like the whole undercarriage was covered in something. I came across a video and discovered the stuff that the undercarriage of my 4Runner was covered with.

https://youtu.be/nXvl9nt57Kg?si=whUEw7Zzq16AccQi
It turns out that all these years I have been fighting to get layers of rubberized undercoating off of the undercarriage.

***​

What is interesting is that after the last application of Fluid Film, layers of dried up rubberized undercoating that I was unable to get off before began to peel off. Fluid Film penetrated all the way down to the frame and released them completely.

While I was researching different undercoating methods, I came across these two videos. Once of them describes how to undercoat with used motor oil, and the other one shows what happened after one year. The second video shows the same process that my frame is going through right now.

https://www.youtube.com/watch?v=HOQbQwustKo

https://www.youtube.com/watch?v=WXiZRjuxeJk

***
​

Initially, I wanted to coat Fluid Film over the previous layer. After discovering that new layers of that junk began to come off, I spent the whole morning cleaning out what was remaining of it.



I didn't bother cleaning off previous layer of Fluid Film that was covered with a layer of dust. I will just coat everything with a fresh layer in a few days.



Looking much cleaner now.

Overall, I am happy with Fluid Film. It protected the areas that were clean initially, and penetrated through a layer of rubberized undercoating and released the remainder of it from the frame. Since I do not live in the Rust Belt, snow and salt are not a major concern to me. I decided if I coat undercarriage once every two years, I should keep it in pretty healthy state.

 

Fluid Film is a good product and I used it on Day 1 to spray inside the rails. However, for this specific case consider using Krown. The reason I am recommending this is that Krown is more liquid and will do a better job penetrating. Despite your efforts to remove old coating, I don't think you removed enough of it, especially in hard-to-reach places and crevices.

In my case, I made a mistake of limited application of a similar shitty rubberized undercoating to a pan of a classic car I really like. Krown penetrates it and prevents it from drying off completely and starting to trap water and flake off. Now, that car is a garage queen, so I don't know how all of this would behave in a dusty offroading circumstances. Krown alone does absorb dust, but it does not prevent it from providing water-repellent protection; but when you start mixing and have existing rust under it... who knows.

 

I will look into that option as well. 95% of all that junk was removed, so it is more about protecting the frame right now.

 

OP, you keep mentioning using "motor oil." The problem with that is that not all "oils" are created equal. For instance, motor oil is formulated mainly to reduce friction inside of an engine. (That's gloriously oversimplified BTW.) Corrosion resistance is so far down the list of requirements as to be inconsequential. Oil based products like Fluid Film, Krown, Woolwax, etc. have a ton of corrosion inhibitors added to the oil base. Their primary goal is to inhibit corrosion. Oddly enough, if used in your crank case, your engine wouldn't last very long because they're not great lubricants. Does that make sense?

 

Yes, sir.

 

Don't use motor oil, it was a thing before we had better options. The only people thinking its a good idea are remembering back 50 years ago before there were the options we had now. It fucks up the environment and isn't very effective, which is the worst of everything.

I personally think paint is the best long term item. I've used the Rustoleum black enamel with a good metal primer and good prep (rust removal). All you need to do is touch it up yearly as it chips.

Fluid film is good in combination or on it's own if you can live with the dirty looking frame. Doesn't last as long, but it's about as effective as paint.

I learned recently about Naval Jelly rust remover, it's like evaporust that works upside down. I think that will be extremely useful for frame prep, and I'm going to use it next time I inspect the frame.

 

I like how this claims to be a restoration but it's all just hacks

 

I think you are right. Never thought about that part. Sometimes it helps to have editors. Will be corrected per request.

 

I know there are lots of threads about this issue on Tacomas. I have one of those, too. I sprayed it down with a cosmolene based product in late 2019 when I got it and it has been through 5 Iowa winters. Iowa gets a lot of snow and ice usually and corrosive products are applied to roads anywhere from October through March usually. My truck's frame looks outstanding. When I got my 4Runner in late 2022, I immediately sprayed it. It has been through 2 winters now and it also looks great. I only reapplied in areas that I obviously missed or like LCAs where they just seem to get blasted. Best practice would probably give it a touch up every spring but it works really well. I've used both RP-342 and CRC successfully. Even if rust on the exterior of your frame isn't as important as internally, it still looks awful so I spray the outside down well.

 

Did a lot of research on this though I’ve never used this product myself. I’m ordering from Amazon as we speak. Thanks for the great walk through and posts!

B’laster surface shield!!!!!


Repair geek

https://youtu.be/NE1_EiZIVHs?si=L5Eg5reejivQAlXc