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Filling Trim Holes in Car Body: Sheet Metal Welding Tips

1/20/2023
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Tags: Tech, Tech

There are many reasons to permanently remove trim from a vehicle, whether it is because some of the original trim is missing, impossible to find, damaged, or just because you want to, trim removal is a common aspect of building a custom vehicle. Unfortunately, far too often driveway builders will simply wipe some filler over the holes and call it a day, only for the next person who owns the car to find cracked body filler or filler worms inside the doors and have to damage the paint to fix it right. Don’t take half-measures, repairing holes in auto body is not difficult and a really good way to practice the art of sheet metal welding. Filling holes in sheet metal is easy with Speedway Motors welding discs for filling holes.

What Gauge is Sheet Metal on a Car?

Sheet metal is thin, even on older cars from the 50s and 60s, the metal was 16 gauge at best, with modern vehicles using as thin as 22 gauge sheet metal for exterior panels. 18 gauge is the most common thickness overall, but this can vary from spot to spot, as all automotive sheet metal is press formed. As the sheet is stamped between two hardened metal dies, it is stretched into shape. This means that in some areas where there is very little shaping, such as large flat sections, the metal should be the original thickness of about 18 ga. In deep draw areas, corners, and valleys, it can be as thin as 20 ga, which is quite thin. Welding holes in metal this thin requires diligent use of the welder, mainly ensuring that your machine is properly set.

The Speedway Motors trim hole patch discs come with several sizes that fit most holes you will encounter. This kit is great for doors, floors, trunks, and firewalls. They come in several thicknesses too so you can plug holes in sheet metal easily.
We used several types of sanding pad for this project with a cordless grinder, but you can use a corded grinder or air die grinder. Plain sanding pads are fine, but the flap-discs really cut down on the heat that goes into the metal, reducing warpage.
Some other tools we used for this project are a triangle magnet, a copper bar and a small body hammer. You also need a welder and the matching safety gear.
How to Weld Holes in Sheet Metal

Sheet metal filler plates are required for holes larger than 1/4-inch. Small holes present a challenge to the hobbyist builder, as they are often too small to fit in a patch panel, and it is very easy to cause warpage. Filling screw holes in sheet metal can be as simple as a single tack weld, though some builders prefer to use welding discs filling holes. To demonstrate best practices for repairing holes in auto body, we had a door from a 1956 Chevy truck and shaved the door handle, mirror mounts, and a pair of holes that had been drilled for a set of “peep mirrors.” For the larger holes, we used a Speedway 14-ga trim hole plug kit, and the smaller holes were welded up without any filler metal at all with a copper back-up bar.

On the 56’s door we know we are not using the giant factory mirrors, so those holes were filled first. We used a surface prep pad to remove the old paint and body filler to get to clean metal. The smallest patch discs fit perfectly.
We tacked them in place, choosing to let the disc sit low in the hole. This gives us the ability to smooth out the welds and not have a raised area on the panel.
Here is the process of welding the sheet metal filler plates in one image- #1 is tacked 180-degrees apart. #2 shows how we continued the stitch on both sides. #3 is the finished repair. This minimizes heat and warpage.
Then we used the flap disc to smooth out the welds flush to the base metal.
How to Fill Holes with a Wire Welder

There are some tricks to welding thin sheet metal. Proper welder setup is critical. We are using a Millermatic 211 AutoSet MIG welder with .023 wire. The thinner wire is best for sheet metal. Because we are using an extension cord (240v), we adjusted the AutoSet wire setting to .030 wire. This tells the machine to use a higher voltage setting, which makes up for the longer power cord. We learned this trick from Miller’s engineers. We set the machine to the small side of the 14-ga range. This provides enough voltage and wire speed to properly weld the 14-ga plugs to the thinner 16-ga door skin without burn through. In fact, we only had a couple of spots where our welds actually caused the door skin to burn through, and it was probably just too thin on the edge of the hole. Make sure you follow the settings for your welder, wire size, and gas type. When welding two different thicknesses together, use the smaller end of the range for the thicker metal. Always ensure your safety when welding, this means gloves, a welding helmet, and respiratory protection. The recent pandemic means you probably have a ton of old fabric masks or gators lying around. These are excellent for reducing the inhalation of grinding dust and particles.

On the top of the door, there were two small holes, too small for the plugs in the kit and we didn’t want to make them larger because they were too close to where the door rolls out. We used the copper bar here, which was clamped behind the holes.
Small holes can sometimes be filled with one weld, as we did here.
On the backside of the upper door holes, the welds look like this. They are flat, but still need to be dressed down with a grinder. The whole door will be media blasted before any bodywork, so don’t stress the rust.
The small bolt hole for the door handle was also welded up. We filled it with three tack welds and while it was still hot, tapped it with a hammer to lightly set it down into the door skin, eliminating the potential for a raised spot.
Once the weld was ground smooth, you can see the very slight depression which can easily be smoothed over with a small swipe of body filler. This will blend out very well.
How to Patch Holes in a Car Body

Technique can make up for setting variances. Starting the arc on the thicker metal and then pulling to the thinner metal helps you control the heat on the thinner metal so you don’t burn through. This may seem tricky at first, especially on a small hole with a single stitch tack, but once you get the hang of it, it is super easy. Stitch welding is the required method for sheet metal, which is welding a series of small tack welds or short beads skipping a few inches, then another stitch bead, skip a few inches, another stitch, wash, rinse, repeat until the entire seam is welded. For small patch holes in metal like we are doing here, you limit it to a single tack, move to the opposite side, tack, etc. The idea behind stitch welding is to reduce the heat build up in the sheet metal, to limit warpage. If you have an air compressor, you can also use a blow gun to quickly cool the metal off. The smaller the hole, the less effective small stitches are. In the case of filling body trim holes where you are not using filler metal, backing up the hole with a copper bar helps to absorb the heat from the weld so it doesn’t soak into the sheet metal and provides a surface to lay down some weld to fill the hole. The MIG wire won’t stick to the copper, and you get a clean inner surface instead of a glob of weld on the other side of the hole.

The best way to fill auto trim holes is practice. Even the best welders have blowouts when filling trim holes in a car body. Burn through is a great teacher for this task, as you have to figure out how to fix the hole without causing more burn through. It will also show you where hidden rust may be lurking that you didn’t know about, as thin rusty metal doesn’t weld up very well.

The biggest part of the project is the door handle hole. On the 56, the handles have a large 1.25” hole with a ½” wide tail. We could have patched this with two discs, but the large hole would require resizing one disc, so we opted for a one-piece patch.
The center lines and tail lines were marked on the panel to facilitate modding the largest patch from the kit to be a perfect patch piece.
Using a fender washer that perfectly fit the hole as a guide, the patch disc was marked to match. You may be asking “why not just use the washer?” It is galvanized, and galvanized metal turns into highly toxic gas when heated to welding temperature.
Then we cut the patch on the bandsaw. You can use a grinder if you don’t have a bandsaw.
We finished off the shaping of the patch with a belt sander. Use some vice grips to hold the patch because it will get very hot.
How to Fill Holes in Metal

You may have concerns about the technique required for different areas of the vehicle, such as how to patch bolt holes in a truck bed vs. floor pans or firewalls. The reality is that there isn’t really much difference between the locations, but there are some considerations you might think about. If the back side of the hole will be exposed, such as on the upper door holes we filled on the truck door, then you may adjust your technique to account for that. You wouldn’t want to use a recessed plug, where the plug is placed on the underside of the hole and welded in. This would be fine on a firewall or floorpan (patch placed over the hole on the interior of the car), because the plug would never be seen.

Once done, the patch is nearly a perfect fit. We held it in place with a triangular welding magnet.
Two tacks across from each other were made to set the patch in the door.
Then we stitch welded the rest of the patch until it was completely welded in. Note the bluing on both pieces. The smaller the heat ring on the base metal, the less warping you are going to have.
All the welds were dressed down with the flap disc until the welds were flush with the base metal. Very little body filler will be required for these, as they are smooth and not warped.
To protect the fresh welds, we sprayed some rattle-can body filler over each repair. We also primed the backsides as well.

Repairing holes in auto body is a good way to practice welding sheet metal before doing larger repairs such as sectioning in quarters or patch panels. When learning how to weld sheet metal on a car, any mistakes made are smaller and therefore easier to fix in the bodywork stage. You will warp the sheet metal, just expect it to happen and it can be addressed. Just be patient, take your time and prep the metal properly and you will end up with a clean repair that you will love when it is all finished.

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