How To Fix Rust On A Car
March 29, 2021
If you live somewhere that actually has a winter, accumulating rust on your car is almost inevitable. Even if you don't live somewhere with a winter, rust is somewhat of a natural occurrence. If you store your car in winter, you might never experience underbody rust, but when you're driving behind semis on the highway and they kick up gravel, then you might have a problem. You might have also just bought a car with a few rust spots.
Whatever the case may be, you want to repair it DIY style. Depending on the severity of your rust, fixing it can easily be done in the garage at home with a few supplies.
The process of repairing rust changes dramatically depending on how severe your rust is.
If you have giant holes in your floorboards or in your fenders, chances are you either need to upgrade to a newer car altogether or you'll need to have professional work done by a body shop if you don't have the resources or knowledge to fix it yourself, which can be very pricy. And you gotta save that money for new wheels and tires!
If you only have surface-level rust or even scale rust, the process isn't as bad and can easily be done in the driveway or garage at home.
As a disclaimer, we will only be discussing repairs for surface level and scale rust since these are by far the most common and can usually be fixed at home.
Also, there are many unique circumstances that may make this process more complicated (or possibly less complicated), so make sure you do more research, inspect how bad the rust is on your car and/or reach out to professionals/an auto body shop about welding new sheet metal in and making major repairs like that.
Image from CarThrottle
How Rust Begins
Rust is a chemical reaction between iron, water, and oxygen that corrodes the surface of metals. And what do yah know, cars are literally one hunk of metal.
Rust often begins from rock chips or from salt and similar elements over time. When bare metal is exposed to saltwater and air over time, chemical reaction and corrosion, aka rust, begins and spreads quickly.
If you own a Miata or some other convertible, it's pretty common for rust to eat away the car starting from the inside due to leaking roofs. Be aware of this if you are a Miata or convertible owner!
Typical Rust Spots To Be Aware Of
As we all probably know, there are certain spots on a car that seem to easily accumulate rust more than others.
- Rear quarter panels and fenders
- Rear bumper
- Along the sills / Rocker panels
- Around the headlamps (also the A-pillar)
- Inside the doors and below the doors
- Front of the hood (from rock chips)
Types of Rust
There are three main types of rust that your car could experience: Surface rust, Scale Rust, and Penetrating Rust.
Surface Rust: Rust that forms on the surface of your car's paint | Severity: Not bad, Easy to fix
Scale Rust: Rust that continues to corrode the surface of your car's paint and starts to flake off | Severity: More work involved to repair
Penetrating Rust: This rust completely penetrates the surface of your car and requires major repairing (welding new sheet metal etc) | Severity: Hard to restore and requires more materials, time, and money
How to Prevent Rust
Preventing rust is actually pretty simple and won't cost you much. If you live in Wisconsin like us and daily your car during the winter, you'll definitely want to go through a carwash at least once a week with the underbody flush to rise all the salt off. Just pay the extra $2, you'll save yourself an annoying rust repair in the future.
Once you get a rock chip on your hood or anywhere on the surface of your car, take care of it asap. Apply some rust reformer and use a little brush to apply your car's paint so there is no bare metal exposed. If you do it early, you're going to save yourself time, money, and stress. I can promise you that.
Some people go as far as putting "bras" on their front bumpers/hood to prevent rock chips.
Another preventative measure you can take is to have the underbody of your car, as well as the wheel wells, sprayed with rubberized undercoating. This will prevent rust from starting and will keep your underbody always looking fresh.
You can do this yourself or you can call up your local shop. This is such an easy way to prevent rust, so I highly recommend doing this.
All The Supplies You'll Need
Here's a complete list of the supplies you'll need to repair surface to scale rust on your car.
- Acetone or adhesive remover
- Lots of sandpaper (40-80 grit, 180-200 grit, and 1000-2000 grit)
- Sanding block (recommended)
- Drill or angle grinder (optional)
- Body patches (if necessary)
- Rust reformer (highly recommended)
- Underbody rubberized spray (if you want to prevent rust)
- Spot putty (optional)
- Body filler (fiberglass strands filler and 2nd smooth filler)
- Spray paint (make sure it matches the paint code of your factory paint)
- Clearcoat paint
- Polish + microfiber clothes or an attachment for a drill
How to Look Up Your Cars Paint Color Code
Here's how to find your car's exact paint code so you don't drive around with blotchy and discolored fenders and body panels.
Driver's Door Jamb
Most of the time, your car's paint code will be on the inside of your door on a sticker. It will usually look like "Paint: XXX"
Another place you could check would be inside the glove box. Same deal, it'll say "Paint: XXX"
Passenger Wheel Well
You might also find the paint code in the passenger side wheel well...
Strut Tower or Firewall
Or the strut tower and on the firewall in your engine bay.
Step 1: Remove Rust
The first step to repairing rust is to well...remove the rust. This requires an array of sandpaper starting anywhere from a 40 to an 80 grit, then moving to a 180 and 400 grit, and later in the process, a 15020-2000 grit. Before you actually start sanding, make sure the surface you're working on is clean of any debris. You can clean it with acetone or adhesive remover or whatever is on your shelf at home.
The idea in this first step is to remove all the rust and bubbled-up paint to reveal the bare metal. Make sure to sand a little bit above where the actual rust ends so you ensure all the rust is removed.
This can simply be done with lower grit sandpaper and a sanding block from your local hardware store or if you have power tools, specifically a drill or an angle grinder, you can use these to get the job done quicker. A block of wood will work well too honestly.
Be patient and persistent with this part. Depending on the severity of your rust, you may be sanding for a while.
Once you get the visible rust on the outside of the car sanded down to the bare metal, you'll want to check the other side of the surface and sand down any rust. For example, if your rust is on a fender, check inside the wheel well and sand all that rust down too.
If you skip this part, the rust on the inside of the wheel well will eventually eat through and you'll have an even bigger problem with more money and time involved. Just follow the mentality of "Do it right the first time" and you'll be good to go.
Before you move on to step 2, spray the clean, bare metal with a rust reformer to help prevent it from rusting again in the future.
Step 2: Apply Body Filler
Once the surface of your car is free of rust and you can see the bare metal, this is when you'll want to get ready to apply a body filler, specifically a fiber strand body filler. If you have actual holes in the body (not major holes), you're going to want to get adhesive body patches to help strengthen the surface in combination with the body filler.
Image from ChrisFix
Now, if you just have minor surface rust, you may not even need to apply filler. In this case, skip to Step 4.
Image from ChrisFix
The body patches you'll get for any holes look similar to chicken wire and are usually stainless steel. These patches will create a surface for the body filler to actually adhere to. The biggest thing with this step is the fact that it restores strength to that surface on your car.
You'll need the fiber stand body filler AND a separate filler that will create a smoother surface. The fiberglass filler will be applied first and once that dries and is sanded down, you can apply the second filler and repeat the process of sanding.
Refer to the instructions on the body filler from the manufacturer as far as mixing the body filler with the harder. The general rule of thumb is one pea size of hardener per golf ball size of body filler. Another way to get the perfect ratio is to make a circle of body filler 1-inch tall. It doesn't matter how large the circle is, it just needs to be 1-inch tall.
Then, squeeze a bead of the harder across the diameter of your circle.
You'll want to build up multiple thin layers of the fiberglass body filler. Once you apply this first filler, let it dry, and sand it, you'll go in with the second filler. Repeat the same process with building up thin layers. Build up the filler taller than the surrounding surface so you can create an even surface when you sand it down.
When you're sanding down the body fillers, make sure to account for any body lines that your car may have.
Also, this step is optional, but if you want to go all out and do it as the professionals do, you can apply a thin, even layer of spot putty. This will fill in any pinholes that you can see. Then, sand this down with a higher grit sandpaper.
Step 3: Sanding
I know that I already mentioned sanding in Step 2, but use this step as a reminder to make sure the surface of your body filler is very smooth and even. You want this area to perfectly blend into the rest of the car's original, good paint.
When sanding down the body fillers, you'll want to use lower grit sandpaper. Once you've worked your way down a little bit, you'll move up to higher grit sandpaper, around a 180 to 200 grit.
Make sure that the surface is completely seamless because any bumps or holes will show once you apply paint to the surface.
Step 4: Paint
This is the more exciting part and requires less manual labor compared to sanding; this is where the primer and paint come in. Don't just buy spray paint that looks similar, make sure it matches because you'll end up creating more work for yourself and it will be very noticeable if the colors don't match.
Refer to the "How to Look up Your Paint Code" above if you don't know how to find your paint code.
A quick tip when spray painting the surface of your car is NOT using painter's tape to secure the spot you're painting. Instead, use paper that's folded hot-dog style and tape it around the area you're working on. This will create a soft line so it's less obvious you painted that area.
Before you apply the actual paint, make sure to spray a good coat of primer. Once you've sprayed about 3-4 coats of your actual paint (start with a thin coat and build your way up), you'll move on to the clearcoat. This is the same process as the colored paint; start with thin coats and build it up.
Image from ChrisFix
Step 5: Wet Sanding, Polishing, and Other
Once your paint has fully cured and the surface is clean, you can either wet sand the surface and apply polish, or skip the wet sanding if you don't think it's necessary and go right for the polish.
This will give your car that shine it once had and complete the repair. You'll feel much more satisfied if you take the extra time to clean your car up real nice like this.
Repairing rust can seem like a lot of work, and it is to some extent depending on how severe your rust is, but generally, it's not as magical and difficult as some make it seem. It takes one trip to the hardware store, or two, and some elbow grease.
The biggest takeaway is to fix small rust spots early, so you don't have to worry about dealing with an even bigger mess in the future. Sometimes rust is inevitable, but if you go through the carwash or wash it yourself at home and/or have the underbody coated with a rubberized coating, your car will be just fine.
Have you ever repaired rust? Let us know your tips and tricks in the comments! Feel free to let us know if you have more questions about repairing rust on your car.