Filling Trim Holes in Car Body: Sheet Metal Welding Tips

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.

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.

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.

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.

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.