Underbody corrosion is not just a cosmetic defect, but a serious threat to the safety of the driver and passengers. When holes appear underneath the underbody, the vehicle loses its structural integrity and exhaust fumes can leak into the cabin, which can be deadly. Owners of old cars are often faced with a dilemma: to send the body to a service center for expensive repairs or to try to restore the metal themselves.
Do-it-yourself semi-automatic welding (MIG/MAG) is the most affordable and effective way to solve this problem in garage conditions. Unlike electric arc welding with stick electrodes, semi-automatic allows you to weld thin automotive metal without the risk of burning through it. The process requires some skill, but if the technology is followed, it produces a durable and airtight seam that can last for many years.
In this article we will analyze in detail all the stages of preparation, equipment setup and the actual brewing process. You will learn how to choose the right shielding gas or flux-cored wire, what current to weld body iron with a thickness of 0.6–0.8 mm, and how to ensure the durability of the repair by preventing rust from reappearing after a couple of months.
Preparing the workplace and assessing the condition of the metal
Before picking up the burner, it is necessary to conduct a thorough inspection of the damaged areas. External rust is often just the tip of the iceberg. It is necessary to tap with a hammer or awl all suspicious places around visible holes. If the metal crumbles or bends under light pressure, it must be removed to a clean, hard layer. Stripping should be carried out with a margin of 2–3 cm from the edge of the visible damage.
The workplace must be perfectly clean. Dust, dirt, oil and residues of anti-corrosion compounds during welding will turn into slag, which will make the seam porous and weak. Use angle grinder (grinder) with a petal disk or a metal brush attachment. Degreasing the surface with a solvent or white spirit is a mandatory step before starting work.
⚠️ Attention: Never cook on surfaces coated with bitumen mastic or anticorrosive without first cleaning it. When heated, these substances release toxic gases and ignite, and the weld will be defective due to contamination of the weld pool.
If you plan to cook on a lift or pit, make sure there are no flammable items under the vehicle. Sparks from semi-automatic scatter several meters. It is also important to ensure good ventilation, as the welding process produces harmful aerosols.
Selection of equipment and consumables
An inverter semi-automatic machine operating in a protective gas (MIG) or cored wire (FCAW) environment is optimally suited for body work. The key selection parameter is the ability to operate at currents from 30 to 60 Amps. Powerful industrial devices are redundant here and can burn through thin metal. The optimal wire thickness for the body is - 0.6 mm or 0.8 mm.
Using a shielding gas (usually argon-carbon dioxide Ar/CO2 in a ratio of 80/20) gives a significantly higher quality weld than cored wire welding. The gas displaces oxygen, preventing oxidation of the molten metal. Flux-cored wire is more convenient for trips “in the field”, but leaves more slag and requires very careful cleaning of the seam.
Don't forget about personal protective equipment. A chameleon mask is required, as it allows you to see the starting point of welding before igniting the arc. Leggings should be made of thick leather, and clothing should be made of non-flammable materials. Thin metal cooks quickly, but there will be a lot of splashes of hot metal.
To weld the bottom, it is better to use a cylinder with a mixture of gases, rather than pure carbon dioxide. The mixture produces less spatter and a more stable arc, which is critical for beginners.
Setting the welding mode for thin metal
Correctly setting up a semi-automatic machine is 90% of success. For metal with a thickness of 0.6–0.8 mm, it is necessary to set the minimum possible current and the corresponding wire feed speed. The arc voltage should also be low. If the machine has synergic control, select the "Thin metal" mode or set the wire thickness 0.6 mm.
Always check the settings on scraps of metal of the same thickness as the area being repaired. The optimal seam should be smooth, with fine flakes, without undercuts or through burns. If the wire “sticks” into the metal and does not melt it, add voltage. If the metal is burned through, reduce the current or increase the speed of the torch.
| Metal thickness (mm) | Wire diameter (mm) | Welding current (A) | Feed mode |
|---|---|---|---|
| 0.6 - 0.8 | 0.6 | 35 - 45 | Pulse/Min |
| 0.8 - 1.0 | 0.6 | 45 - 55 | Medium |
| 1.0 - 1.5 | 0.8 | 60 - 80 | Medium |
| 1.5 - 2.0 | 0.8 | 80 - 100 | Max for body |
It is important to consider that voltage and current strength affect the shape of the seam. At low tension the seam will be convex and narrow, at high tension it will be flat and wide. For the bottom, it is better to strive for a moderately flat seam with good edge penetration, but without burn-through.
What is “burn-in” and how to avoid it?
A burn-through is a through hole formed due to overheating of the metal at one point. To avoid it, do not hold the torch motionless, constantly move it, taking breaks to cool the welding area.
Through-hole welding technology
If a through hole has formed in the bottom, you cannot simply “fill” it with wire - the metal will flow down. First you need to install a metal patch (patch). Cut a piece of metal slightly larger than the hole and spot weld it around the perimeter. The distance between the points should be 1–2 cm.
After tacking the patch, you can begin filling the seam. Move the torch in short back-and-forth movements to allow the metal to cool. Do not try to weld with a long continuous seam - warping metal is inevitable. Cook in 2-3cm sections, moving from one edge to the other to evenly distribute the heat load.
☑️ Algorithm for welding a hole
Pay special attention to the edges of the hole. Penetration should occur with 1–2 mm of base metal being captured. If the gap between the patch and the body is large, it can be filled with welding material, but it is better to fit the parts tightly. Use a copper pad under the hole if there is nowhere to put the patch - it will not weld to the copper, but will allow you to form a bath.
⚠️ Attention: Do not overheat adjacent areas of the body. If the metal turns red from the heat, stop and let it cool. Excessive heat changes the structure of the steel and can lead to cracks in the future.
Cleaning and anti-corrosion treatment of the seam
After completing the welding work, the seam looks unpresentable and, more importantly, is vulnerable to moisture. Welding slag (if flux-cored wire was welded) must be knocked down with a hammer and cleaned with a brush. Then the seam is sanded with a grinder with a flap wheel or sandpaper until smooth. It is important not to overdo it and not to thin the metal to a dangerous minimum.
The most important stage is rust protection. Just painting the seam with paint is not enough. Use special primers with a rust converter if micro-foci of corrosion remain. Then apply epoxy primer, which creates an airtight film that keeps oxygen and water out.
The finishing layer for the bottom is special mastics based on bitumen or rubber. They have sound insulating properties and are resistant to gravel impacts. Apply the ointment in a thick layer using a brush or spray, covering not only the seam, but also the surrounding area.
The quality of anti-corrosion treatment is more important than the welding itself. Without reliable protection, even a perfect weld will rot in one winter.
Common mistakes when welding yourself
Beginners often rush and try to weld a large area in one pass. This leads to body deformation (“waves” on the metal) and burns. Another common mistake is poor stripping. Rust under a layer of new metal continues to destroy the structure from the inside, negating all the work.
Using the wrong wire, such as wire that is too thick (1.0mm or more) for a thin bottom, will make it impossible to create a clean seam. The arc becomes too tight and control of the bath is lost. It is also a mistake to ignore the reverse side of the seam. If access allows, the welded area on the reverse side also needs to be cleaned and treated with anti-corrosive.
- 🔥 Overheat: Cook in short bursts with pauses to cool.
- 🧹 Dirt: Clean the metal until it shines, degrease the surface.
- 🛡️ Protection: Don't skimp on anticorrosion, use a multi-layer system.
- ⚡ Settings: Select the current and voltage on the scraps before the main work.
Is it possible to weld a body with a regular electrode?
Theoretically it is possible, but it is highly not recommended. It is very difficult to control the penetration of thin metal (0.6-0.8 mm) with an electrode (MMA), and there is a high risk of burns and severe deformation. The seam is rough and requires significant cleaning. For the underbody, semi-automatic (MIG) is the only choice for garage repairs.
Do I need to remove the fuel tank before welding?
Yes, if the work is carried out in close proximity to the tank or fuel lines. Sparks can burn through the plastic tank or damage rubber hoses, resulting in a fire. Safety comes first: it is better to dismantle interfering elements than to take risks.
Which gas is better: pure argon or a mixture?
Pure argon is not suitable for welding black steel (car body) - the weld will be porous and unstable. Pure carbon dioxide (CO2) is acceptable but produces a lot of spatter. The best option is a mixture of argon and carbon dioxide (80% Ar + 20% CO2), which ensures a stable arc and minimal spatter.