Repairing a Rust Hole in a Car





Introduction: Repairing a Rust Hole in a Car

My husband's car had developed a large rust hole in the fender under the gas cap. 

It needed to be fixed before the winter, and my husband hadn't had the free time to repair it himself; and I didn't want to pay the $150-200 to have it done by a body shop.

So I figured I'd do it myself and surprise him.

Step 1: Scrape and Sand

The first step was to scrape off the loose paint surrounding the rust spot, and to sand off most of the rust.

I found, under the outer layer of metal, what I thought were additional layers of rusted, chipping metal, was actually dense foam padding. I scraped the rest of the metal chips off of the foam. I then taped off the area, and sprayed it with a rust treatment.

Following the directions on the can, I allowed the rust treatment to remain on overnight before I continued. (Rust treatment goes on clear, dries black.)

 On day 2, I again sanded around the area, removing paint around the hole to prepare it to be built up. For rust and paint removal, I used a 60 grit sandpaper.

Step 2: Building Up (part 1)

At this point, I used a metal mesh body patch (bondo brand) to fill the hole, replace the missing metal, and give the bondo a place to adhere to.

Using regular scissors, I cut the mesh to approximately the right shape, and then began fitting it into the hole between the metal and the foam. Along the bottom, I had to loosen the foam from the metal a bit using a screwdriver. Since the area is curved, I had to shape the patch as I worked, and getting the last (upper left) corner in took some work.

The body patch is self adhesive, and has a removable backing. You can see in the photo where it crinkled up while I was maneuvering the patch into place. According to the instructions, I should have taken the patch back out, peeled off the backing, and then re-inserted it. But once I got it in and shaped, it wasn't coming back out, and I decided that I don't care if there's a plasticy sticker bit in there.

Step 3: Building Up (part 2)

The next step was patching the hole with bondo body filler. This was interesting, as I'd never used the stuff before.

It comes with two parts,  the body filler putty (grey), and the hardener cream (red), which you mix together, and then apply.
The instructions say to use a golf-ball sized amount of putty and an inch and a half of hardener. It said to mix on a non-porous surface, and to use within 2-3 minutes.

A few lessons learned: mix up what you can use in 30 seconds. This stuff dries fast, and isn't workable once it starts drying. After the first batch, and trying to clean my non-porous surface, I switched to a half batch mixed on wax-coated paper plates that could be thrown out. 

Apply. Allow to dry. Sand. Repeat until you're pleased with the patch.

Because the rust hole was nearing the ledge under the gas tank door, part of my repair plan was to run the repair up under the plastic edge, to protect the repair from gas or water. This made application and sanding a little tricky, but I'm pleased with the result. Hopefully, the extra work will make the repair last longer.

I allowed the patch to cure overnight before beginning the painting process.

Step 4: Primer

The next step was to apply a primer over the patch. 

I used a self etching, sandable primer, and followed the instructions on the can. I taped over the edges while I was working. Once I was done, I used a 600 grit sandpaper to soften the edge of the primer, and roughen the surrounding paint.

That's me sanding. Photo by my wonderful husband, who was not at all worried about his wife attempting auto body repair. ; )

Hand sanding takes a bit of extra time, but allowed me extra control. Here you can see that I made sure to get the primer up under the lip of the plastic. I also kept tape and newspaper over the lower (plastic) fender, as I didn't want to scratch that up while sanding the bottom edge of the metal fender.

Step 5:

The final step was painting. The first color I used wasn't right. It looked right on the can top, when looking at the car from a few feet away and standing up; but once on, was much too light.

I picked up the next shade darker, and it matched when I placed the spray can cap against the bottom of the car, with no sunlight reflection to skew the color.

The last step was to apply a clear lacquer coat over the color paint.

Lessons I learned about painting with spraypaint:
1. Light coats, sprayed quickly
2. Don't reverse direction. Spray across, stop while moving. Spray the opposite direction, stop while moving. The slight buildup on reversing directions leads to drips that have to be sanded off and re-done.
3. The lacquer says two light coats and then a wet coat. "Wet coat" does not mean "heavier coat". A heavy coat results in bubbles. Which is a pain to remedy when you were nearly finished.

Step 6: Final Result

So there you go. My first attempt at auto body repair.
It turned out pretty well, in my humble opinion. I learned a lot. It cost me about $60 in supplies, and some of my time over the course of three days.
My husband's pretty pleased about my little present to him, and I'm proud of myself for figuring it out.



    • Pocket-Sized Contest

      Pocket-Sized Contest
    • Pro Tips Challenge

      Pro Tips Challenge
    • Paper Contest 2018

      Paper Contest 2018

    We have a be nice policy.
    Please be positive and constructive.




    Wow. Well, that's a little more than I would've tackled...before seeing your project! That's pretty cool and for a first-timer, great job! I need to do some body work on my car but it's fiberglass parts. I wouldn't mind doing it myself to keep from paying uber amounts just for looks. Maybe I will try it thanks to you. :)

    cal_gecko - the site has a "...'be nice' comment policy. Please be positive and constructive with your comments or risk being banned from our site. " I don't believe your post was any of these...

    Another commenter suggested a product called Kitty Hair for fiberglass. He says it works better than bondo, even on metal. You might want to check that out or mssg him. It looks neat!

    i have a question what did he use to sand off the rust and what brand of rust treatment did he also uses please response back i need to know immediately please and thank you.

    I will check on that! Thanks. You wouldn't want a fiberglass-fix practice car would you? Lol.

    Ah, for lack of free time.... ; )

    Great work. Very well written and done.
    You SHOULD be proud.
    How many husbands have such a smart and capable wife?
    More women should be willing to know how to fix things.
    You can't always be sure there is a man around to do it for you.
    And of course, the 'retail' market for fixing things (if you can even find anyone to really fix something) is absurdly costly.
    We are a throw away society and will soon drown in our own junk.
    Good for you on this well done instructable.
    Let's hope it encourages more women to follow suit.

    Thank you, CedricWard!

    Not only is this lady's initiative, native good sense and intelligence commendable, but she did a good job with what she had to work with ... an older car.

    Also, this instructable was very well written, she should be writing manuals! Excellent job.

    One comment .. I think she underestimated the cost of a "professional" job by a factor of 3 to 5, depending on how "professional" the professional is. It costs soooo much money to get body work done.

    ps. My husband and I owned an auto repair shop for over 20 years, and we made a lot of money fixing the repairs done by "professionals".

    thank you so much, cwickenkamp!
    that means a lot coming from someone with 20 years of experience!!

    You are welcome. It's only the truth. :-)