The Detroit Post
Thursday, 21 October, 2021

How Repair Rust Holes

Brent Mccoy
• Thursday, 05 November, 2020
• 8 min read

At the very least, it’s unsightly and reduces your car’s value; if left unchecked a rust spot can quickly grow until it causes a structural problem. In many states a car with rust holes in the body won’t pass inspection, regardless where the spots are located or how serious they are.



For these and other reasons, if your car has body rust it’s best to deal with the problem, because it’s certainly not going to go away on its own. While patching a hole caused by rust is not too difficult, it does take some patience, especially if you want to repaint the vehicle yourself.

If the hole caused by rust is not in a load-bearing area of the car, such as the chassis, it is possible to repair it with glass fiber or body filler. This will make your vehicle look whole again and prevent the rust damage from spreading.

With that in mind, here’s what you’ll need to do to remove a rust spot and patch it up nicely. Body filler kit Breathing mask Electric drill with wire brush or sanding disc attachment Goggles Hammer, ball peen Paint primer Paint Protective goggles Rust resistant primer Note : Chemical rust remove is not necessary, but can help ensure that you’ve gotten all the rust, which is absolutely necessary if you don’t want it to return.

It’s also possible to use this instead of a grinder, if your rust isn’t extensive; follow the manufacturer’s directions. Your parts store can help you find the right paint for your car.

You’ll need an appropriate space to do the work, which ideally means a garage or similar structure, as you won’t want to be outside once the painting begins. The garage should be well lit and well ventilated, and anything that can’t get dirty should be removed, as what’s coming is not a clean process.

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If you don’t own these tools, you can rent them at many large hardware stores. Be sure to get sanding discs (80-grit is best for metal, 120- or 150-grit for removing paint and shaping patches) and a grinding wheel too.

You’re going to be throwing sparks around with your grinder, and you must protect your eyes, so get some safety goggles. This repair involves a lot of airborne dust, which can do serious damage to your lungs.

Even better would be to buy or rent a respirator, because you’ll also be working with quite a few toxic chemicals. Other clothing : Long pants and a long-sleeved shirt are a good idea for this repair, even in hot weather.

You’ll want a good pair of work gloves, which will help you avoid cutting yourself on the sharp edges of the panel and will protect your skin from the not-so-nice chemicals you’ll be working with. If you’ll just be using primer, mask as much as possible, leaving only a few inches to each side of the rust spot.

Even if you’re not planning on priming, it’s best to mask nearby panels because flying sparks can damage your paint. Be sure to cover other parts of the car, including nearby tires, glass, and chrome.

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A heavy drop cloth will do; if all you have is newspaper, use many sheets to form a thick protective layer. Anywhere from 80 down to 150 grit sandpaper works fine for this, preferably with a sanding block or, of course, a mechanical sander.

Don’t stop until you’ve cleared the paint from the entire rust spot, and back far enough that you can be sure you haven’t missed anything. It is absolutely critical that you leave no rust, so don’t stop until you’re down to bare, shiny metal at every edge.

Don’t worry too much about breaking through, even if the rust spot wasn’t all the way through to begin with, as you’ll be fixing that with your patching kit. The protective coating seals the surface against any new moisture which helps prevent further rust.

Apply per the manufacturer's instructions and wait the recommended amount of time before moving on to the next step. If necessary, tap the edges of the hole inwards with a small ball-peen hammer down to the level of the body panel.

Body filler can be purchased as a kit at any auto supply store. Most kits contain a body filler paste, a hardener and a piece of expanded zinc metal, or fiberglass cloth which is used to support the filler while patching the hole.

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Use the plastic applicator that comes with the kit to apply the body filler to the hole. After the patch is dry, you’ll want to sand it down flat with the rest of the body panel.

Go slowly and carefully, as you don’t want to take off too much of the body panel. If you’re applying primer (and you should if you don’t want the rust to return), you’ll want a smooth surface, so use finer-grit sandpaper, down to 240- or so.

Clean the surface well (it must be free of dust), and apply the second layer of primer. After your final (second or third) coat of primer, you can call it quits or move on to the painting step.

Try to use long, even strokes, overlapping slightly but avoiding excessive thickness or drips. Clean and dry the repair and then repeat the process twice more (sanding in between), until you’ve done three coats.

This can be done using a tin snips or an electric drill with a wire brush, flap wheel or sanding disc attached. The protective coating seals the surface against any new moisture which helps prevent further rust.

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Apply per the manufacturer's instructions and wait the recommended amount of time before moving on to the next step. If necessary, tap the edges of the hole inwards with a small ball-peen hammer down to the level of the body panel.

Warning : Glass fiber can irritate the skin, so it is best to wear rubber gloves when using these kits. Use the plastic applicator that comes with the kit to apply the fiberglass resin to the matting.

For the novice builder, these tasks can mean a costly trip to the body shop or lots of frustration in the driveway. Proper techniques and tools will help make any project easier to accomplish.

Locating a good practice panel on your ride that is less visible is a great place to start. While replacing large sections can be relatively easy, the smaller rust holes that do not warrant a full panel replacement are small enough that the average builder can tackle them and be back inside in time for dinner.

Using an air blow gun to cool the area reduces the heat effect even more. To demonstrate this process, we cut out and repaired some rust bubbles in the trunk lid of a 1971 Buick GS convertible.

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Now a few of these are holes for the badge, the big ones are just rust. A patch panel was made from 14-gauge steel and coated with weld-thru primer to keep it from rusting. Next, we cut out the offending area with a cut-off wheel. We treated this with rust converter and then some undercoating. The scale was knocked off with a wire brush and then treated with rust converter. Once the rust converter had cured, it was sprayed with undercoating to seal it. The panel was fit and then held in place with magnets. Using the stitch-weld method, the panel was welded into place. The last step before body work is grinding down the welds.

A flat grinder works, but a flap-wheel disc cuts faster and does not heat up the metal nearly as much. Take your time and practice before you actually weld on your car. Rust repair is not that hard, but it is really easy to make a bigger problem if you get ahead of yourself and bite off more than your skills can handle.

His career began in the car audio industry as a shop manager, eventually working his way into a position at Rockford Frigate as a product designer. In 2003, he began writing tech articles for magazines, and has been working as an automotive journalist ever since.

Jefferson operates Red Dirt Rod, his personal garage studio, where all of his magazine articles and tech videos are produced. 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. I found, under the outer layer of metal, what I thought were additional layers of rusted, chipping metal, was actually dense foam padding.

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Following the directions on the can, I allowed the rust treatment to remain on overnight before I continued. 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. At this point, I used a metal mesh body patch (bond brand) to fill the hole, replace the missing metal, and give the bond 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.

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.

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 plastic sticker bit in there. The next step was patching the hole with bond body filler.

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It comes with two parts, the body filler putty (gray), 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.

This made application and sanding a little tricky, but I'm pleased with the result. I allowed the patch to cure overnight before beginning the painting process.

Once I was done, I used a 600 grit sandpaper to soften the edge of the primer, and roughen the surrounding paint. Photo by my wonderful husband, who was not at all worried about his wife attempting auto body repair.

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.

The slight buildup on reversing directions leads to drips that have to be sanded off and re-done. My husband's pretty pleased about my little present to him, and I'm proud of myself for figuring it out.


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