How to reduce the risks of welding wafer-thin metals

How to reduce the risks of welding wafer-thin metals

One of the most irritating things about building or modifying a hot rod is cleaning up after the previous owner(s). Your pride and joy has probably been through a long chain of title holders, many of whom drilled umpteen dozens of holes in, on, and around the body.

While holes in heavy-gauge steel are fairly simple to weld shut, perforations in thin-gauge sheetmetal are another thing altogether. It takes a skilled hand to fill them correctly; if done without care, it is easy to do more harm than good. Dale Kutsch of El Monte, California, recently turned us on to a neat little trick that greatly reduces the risk of welding wafer-thin metal.

Thin metal like this are easy to be damaged with bad welding skill
Thin metal like this are easy to be damaged with bad welding skill
  1. There is no better time to make repairs than when the car is apart for service. The firewall and inner fenders of our’s were peppered with little holes that no longer served a purpose, so we enlisted Kutsch to close ’em up. If your car has an electronic-ignition control module, ECM, or EFI unit, remember to disconnect the ground wires so that the electromagnetic field generated by the MIG welder won’t harm these delicate components. Prep the surface by reducing the area to bare metal and removing oily residue.
  2. For thin-gauge work, Kutsch uses a 110-volt MIG welder equipped with 0.030-inch wire stock; the real trick is how he positions a brass plate on the backside of the hole. The brass supports the molten metal as it flows into the hole; it also transfers heat away from the work area to prevent the surrounding area from warping. As the brass absorbs heat, it must be cooled periodically to remain effective. Filling a 3/8-inch hole (as shown here) usually requires one respite; the welding stops, and the brass is dipped in cool water for a few seconds. Dry the brass before resuming the process because water drops can contaminate the weld.
  3. Since brass is nonferrous, it will not stick to the weld. Kutsch prefers a plate that is at least 3/16-inch thick. A thicker one is unwieldy and difficult to manipulate. The small pock marks visible here are the only scars after four years of use. An assortment of brass plates makes hard-to-reach jobs easier.
  4. Here, Kutsch closes holes where a fuel-pressure regulator once was. Note that the brass plate is being held against the backside of the unibody by his left hand as he welds with his right. When it isn’t possible to gain access to the opposite side of a panel, Kutsch feathers the trigger, creating a series of safe, short-duration welds. The idea is to make sure that the surrounding metal doesn’t absorb enough heat to distort or melt. This technique is possible only with a 110-volt MIG setup; a 220-volt stick welder usually creates too much heat. After all the holes are filled, grind the welds flush, and apply a skim coat of body filler before priming and painting.