4Runner Undercarriage Cleaning & Corrosion Prevention – A Preventative Maintenance Guide

5th Gen 4Runner With Morimoto XB Headlights & Gobi Roof Rack

Undercarriage Cleaning & Protection – How Corrosion Forms & How Can You Help Prevent It From Happening

Corrosion is inevitable, there is almost no avoiding it in a natural world. Everything is trying to slowly break down and make its way back to its natural state, even if it takes 1000 years. Unless, there is an outside force (you); who uses techniques, tools, and products to prevent corrosion from taking over your hard-earned rig.

This was something that I wanted to start from the very beginning of ownership of my 4Runner. All of the locations I  work near are by the ocean, so I wanted to make sure that I stayed on top of corrosion prevention. From living by the Gulf of Mexico in Florida to the island of Oahu in Hawaii; I have maintained the undercarriage by keeping it clean, and protected, and stopping visible rust in its tracks.

The primary purpose of this article is to guide you through light to medium correction of troubled areas and prevent corrosion. I started working from scratch on a brand new 4Runner, so it has been easier for me to keep up with preventative maintenance.

The earlier you start, the more effective and beneficial it will be for your rig. Getting a layer of protection everywhere you can reach will help prevent rust from forming on the undercarriage, and help keep it clean.

Some basic steps to follow and keep in the back of your mind as you go should be:

  • Clean the surface
  • Eliminate corrosion
  • Coat & protect the surfaces

Tools & Supplies To Complete Undercarriage Cleaning / Protection


I will list everything required to do this job from start to finish in general. However, I won’t be using all of it thanks to my ongoing preventative maintenance.

The focus here will be on the rear section of the undercarriage. It will take some time if you are doing the entire vehicle. If you can already see quite a few areas that will need some work, do yourself a favor and split up the job. Although doing the entire undercarriage in one day is possible, it depends on how much corrosion needs to be cleaned up.

The corrosion cleaning will be the most time-consuming, and cleaning the entire undercarriage will be a dirty process that can take some time as well.


Helpful Tools (Optional)

  • Pressure washer
  • Creeper


How Corrosion Forms (Simple Explanation)

Undercarriage Corrosion

Corrosion is an electrochemical or chemical process that causes metal to react with its surroundings. In the case of our rigs, we mainly deal with Oxidation. Oxidation occurs when a metal is exposed to oxygen and moisture. Through this chemical reaction, you will see some of the metal corrode or oxidize in the form of rust.

All metals have different properties that determine how fast or slow they break down on an atomic level. With the frame of the 4Runner being iron-based, it is very susceptible to corrosion if the bare metal is able to be exposed to oxygen (air) and moisture.

How Salt Accelerates Corrosion

Salt doesn’t cause corrosion by itself, it just accelerates the chemical reaction.

Without getting into too much science, electrons are able to flow easier through salt water. Salt water would be an electrolyte solution containing more dissolved ions than fresh water. This would allow for the reaction between the metal and oxygen to occur at a more rapid rate.

Tools To Eliminate Corrosion

Undercarriage Cleaning Tools

The following methods are easy to do and don’t cost much money, just your time. There are many other methods out there, but these ones I found helpful due to their cost and obtainability.

Personally, I like to start with the most non-destructive method first, then work my way up to harsher methods. That way, you do as little damage to the focus area and expose more metal than you need to. Your frame is already coated and protected by the layer of paint on the surface. The more paint you remove, the more exposed metal area you have to protect.

The process I usually take to achieve my end goal is listed in the following order:

  1. Rust dissolver 
  2. Scotch pad/sandpaper
  3. Dremel tool 
  4. Wire brush or wire wheel drill attachment

Rust Dissolver

Undercarriage Corrosion 2

I usually start with a dissolver, especially if it’s a smaller area. this helps keep the damage to a minimum. Using a rust dissolver is easy; throw on some gloves and apply with a brush.

This process is just a chemical reaction between the oxidized metal and the dissolver solution. It takes a bit more time and re-applying the dissolver over the focus area might be required.


  • Good for small to medium areas
  • Easy to apply
  • Relatively cheap
  • Great for smaller components that can be placed in a cup (hardware, brackets, etc.)


  • Takes more time
  • Working with chemicals
  • Will only remove light to medium corrosion

3M Scuff Pad/Sandpaper

Removing Undercarriage Corrosion

I use this method for quick jobs or touch-ups before applying paint. Using sandpaper in various grits by hand can help get rid of rust faster than using a dissolver, and is less destructive than a wire wheel or brush. If you are careful enough, it’s also possible to use an orbital sander for even quicker work.


  • Easy to do
  • Cheap to get
  • Effective for small to medium areas
  • Good for quick jobs


  • Can be tiring
  • Can take more time(if done by hand)

Dremel Tool

Dremel Tool

This thing has been one of the most helpful tools in my arsenal. With it being compact and coming with various attachments, I am able to get into smaller areas to clean up corrosion faster and more effectively. If you are willing to fork out some cash, they have a wide variety of options for Dremel tools.


  • Very quick at eliminating corrosion
  • Great for small to medium areas
  • Can get into tighter locations


  • Tools can be pricey
  • Corded, depending on the model you get
  • Buying extra attachments or parts can be pricey

Wire Brush or Wire Wheel Drill Attachment

Power Drill Wire Brush Kit

I use this as a last resort in most circumstances due to how aggressive it can be. Although it is very effective at getting rid of corrosion, it can remove more than you want. Start with a wire brush and if you need more power, grab a drill and wire wheel attachment.


  • Cheap to purchase
  • Effective for small to large areas
  • Less time to cover more area


  • Can take more paint or scuff more metal than desired
  • Can’t fit into tight places with a wire wheel and drill

Methods Of Prevention

Undercarriage Rust Prevention

This section of the article is to protect what you have already that is not corroded. It’s also for the areas that you have already cleaned up or eliminated corrosion from. Below are some easy and common methods to prevent corrosion. Having a routine cleaning schedule is a great way to keep up with the process too.


Paint is the primary and first layer of protection for the metals on your 4Runner. Some surfaces underneath do not have paint, those are different alloys of metals that are more corrosion resistant. In areas on your frame that have taken damage and have not started to corrode or areas that have been cleaned up, paint should be the next step to help cover the un-painted surface

Most paints already have some kind of corrosion inhibitor mixed into the paint. This gives it another layer of protection when you apply the paint to protect bare metal areas. Paint will be your first line of defense.

CPCs (Corrosion Preventative Compounds)

If you don’t need to paint anything, and you are just trying to protect what you already have, this is the next step. CPCs will save you in the long run and help you maintain your undercarriage. Most CPCs come in an aerosol can, which makes it easy to apply.

I use Fluid Film, due to it being able to be found almost at any auto parts store or home improvement store. It is also affordable and comes in an aerosol can. The product has proven to work very effectively. I have been using it since the beginning of my 4Runner ownership. There are many other CPC options, but in 4 years of ownership, there hasn’t been a reason for me to switch yet.

Step 1. Thoroughly Clean Undercarriage

Cleaning Vehicle Undercarriage

Cleaning the undercarriage is going to be crucial, especially if you plan on doing any painting. Not only that, but so you can find areas that need correction as well.

Clumps of dirt or leftover mud will trap moisture and that’s one of the factors that contribute to corrosion. Clean out the inside of your frame the best you possibly can by running water through the frame and letting it drain out. It’s a good idea to remove any skids or armor underneath and clean behind. You want to get everywhere you can possibly reach.

Step 2. Inspect

4Runner Rear Undercarriage

After you have cleaned all the surfaces you can reach, Inspect all the surfaces and find where your undercarriage needs help. Take note of all the work that will need to be accomplished. While doing the inspection I found a couple of areas that were not too bad, and pretty simple to take care of.

Step 3. Eliminate Corrosion

4Runner Undercarriage Corrosion Cleaning

This is where you will work from the least destructive methods up to the most. In my case, I found four to five areas that were similar to the one above, with paint flaking away from the frame. All of the locations I found corrosion were on welds or in a crevice. I made sure the flakey paint was gone and no other corrosion was hiding underneath.

Since they were all about this size or smaller, I opted to just work with the rust dissolver. Once the dissolver has done its job, I scuffed up the area lightly with a 3M pad to prep for paint.

If the rust was not able to come off with this method I would move up to the sandpaper, then the Dremel, and so on. Luckily these were all simple areas that needed correction

Step 4. Protect The Bare Metal Surface

4Runner Undercarriage Corrosion Cleaning 2

Now it’s time to protect the metal you prepared and rid of corrosion. I will be using satin black Rustoleum paint to coat the bare metal areas.

The method I use to paint really small areas like these is using a spare piece of cardboard as a palette. Spray a bit of paint on cardboard then use a brush to dab up the paint. By doing this, you don’t have to worry about taping off such small areas.

Step 5. Protect The Painted Surface

Repainting 4Runner Undercarriage

Once complete with all cleaning and painting, now it’s time to throw a layer of Fluid Film down to protect the finished product. After you have completed all of this, it’s up to you how often you want to inspect and maintain the undercarriage.

Final Thoughts

Fluid Film 4Runner Undercarriage

My undercarriage maintenance schedule consists of doing a full cleaning every six months or so or after heavy off-roading. Full cleaning comprises a start to finish of clean, rid of rust, painting, and protecting.

The only other time is when I detail/wash the 4Runner. After washes, I will spray CPC and wipe down all wheel wells and components, and spray the main frame inside and out, running forward and aft the best I can.

This routine also makes it way easier to clean after going off-roading. Once you have set the standard for your rig, you will be able to come back later and spot-check areas that need help on a smaller level rather than go through the whole process every time.

Corrosion will end up getting to a point where it might be unmanageable to do at home if you don’t stay on top of it.

Overall, it can be hard work and can take some time. However, this could be really important if you plan on selling, or just want to maintain your 4Runner’s appearance. It’s on you to stay on top of the process. Throw on your favorite jams and get to work. You worked hard for your rig and your rig is working hard too, so do her a favor!

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1 year ago

Good article but it looks incomplete. It’d be great to see how to correctly spray CPC, and especially how to maintain the inside of the frame. Thank you.

1 year ago

wow – really thorough. Living at the beach – I use a product called Salt Away. I usually add the recomended amount to my foam cannon and spray the undercarriage and let it soak. would love to know what people think of this technique.

1 year ago

Web Site Request: Can we get a print friendly button to print the article without the other web page content, like you print a recipe wanting just the ingredients and instructions?

Thanks a bunch!!!

1 year ago
Reply to  Bill

Select what you want, Cut and Paste to Word or Editing program of your choice. You can even pate pictures to it. Super easy Computing101.

1 year ago

This article will be bookmarked, good stuff Lane.

Very logical and great set of references, I will refer to this for my maintenance tasks.

Love the content from this website indeed

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