Introduction: Repairing a Rust Hole in a Car

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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.