The appearance of through holes on the body is always stressful for the owner, because we are talking not only about aesthetics, but also about the safety of the structure. Corrosion is capable of turning durable metal into dust in just one season, if you ignore the first “saffron milk caps” on the sills or arches. Many drivers mistakenly believe that covering the hole with putty or epoxy is enough, but this approach only delays the inevitable and makes the problem worse.
Correct repair of through corrosion requires complete removal of rotten metal and restoration of the integrity of the power elements. In this article we will analyze proven techniques, from classical welding to modern polymer composites. You will learn how to prepare the surface, choose a material and apply a coating that will last for many years, and not until the first winter.
Ignoring the problem may result in the car losing its marketable value or even failing to pass technical inspection. Rust through often hidden under decorative trims or in hidden cavities of the side members. Therefore, the first step to success is a thorough diagnosis and understanding of the scale of the disaster you have to contend with.
Diagnosis and assessment of the extent of damage
Before you take up a grinder or welding machine, you need to objectively assess the condition of the body. Often the visible hole is just the tip of the iceberg, underneath which lies a large affected area. Tapping the metal, using a magnet, and visually inspecting with a flashlight will help determine the actual boundaries corrosion center.
Particular attention should be paid to areas where moisture accumulates: the lower edges of doors, internal cavities of thresholds and places where bumpers are attached. If the metal around the hole is easily dented with a finger or a screwdriver, it must be cut out with a margin. Healthy metal should not peel or crumble under mechanical stress.
⚠️ Warning: Never attempt to weld or seal loose, rusty metal. This is guaranteed to lead to the reappearance of corrosion after a few months, since the source will remain inside.
To accurately determine the boundaries, it is often necessary to dismantle the interior trim or plastic elements. Only after making sure that you see the entire perimeter of the damage can you begin to plan the work. Sometimes it makes more sense to replace an entire part (for example, part of a wing or an entire sill) than to patch many small holes.
Choice of restoration technology: welding or composites
There are two main ways to solve the problem: thermal (welding) and chemical (composite materials). Semi-automatic welding in a protective gas environment (MIG/MAG) is considered the gold standard, as it provides a monolithic connection, similar in properties to the factory one. However, this method requires equipment and skills in working with metal.
An alternative for those who do not know how to cook are epoxy composites with fiberglass. Modern two-component compositions have high adhesion and strength, allowing you to create patches that are resistant to vibration and temperature changes. The main advantage is the absence of thermal effects, which is important when repairing complex profiles.
The choice of method depends on the location of the damage. On power elements (spars, suspension mounts), only welding or replacement of parts is permissible. For uncommon areas, such as the bottom of doors or arches, a quality composite can be an excellent solution. It is important to understand that durability repair directly depends on surface preparation.
When using fiberglass, apply the material in layers, allowing each previous layer to cure slightly, but not completely, to ensure better adhesion between layers.
Surface preparation and corrosion removal
The quality of preparation makes up 80% of the success of the entire event. You need to remove not only the rust itself, but also all adjacent metal that shows signs of damage. For this, an angle grinder is used (Bulgarian) with cleaning and flap wheels.
The stripping process should be carried out until clean, shiny metal appears. The boundaries of the transition from healthy metal to the repair zone should be smooth, without sharp steps. This is necessary so that later the putty or weld will lie flat and not create stress points.
- 🛡️ Clean the metal to a shine on both sides of the part, if there is access.
- 🛡️ Treat hidden cavities anticorrosive before installing the patch.
- 🛡️ Degrease the surface with a special composition before applying materials.
After mechanical cleaning, it is recommended to process the edges of the hole rust converter based on orthophosphoric acid, if microfoci remain in hard-to-reach places. However, you cannot rely only on “chemistry” - mechanical removal is a mandatory step.
☑️ Preparation checklist
Technology of welding and installation of metal patches
If you choose welding, then the ideal option would be to use a semi-automatic machine with wire with a diameter of 0.8 mm. Thin body metal is easy to burn through, so it is important to correctly set the current and wire feed speed. Patch must be made of metal of the same thickness as the part being repaired.
Before welding, the patch must be tried on and fixed. The gap between the edges of the hole and the patch should be minimal (1-2 mm). Welding is carried out with short seams (1-2 cm) in a checkerboard pattern, allowing the metal to cool to avoid warping.
| Parameter | Recommended value | Impact on the result |
|---|---|---|
| Wire diameter | 0.6 - 0.8 mm | Allows you to weld thin metal without burns |
| Welding current | 40 - 60 A | Depends on metal thickness and gap |
| Gas | CO2 or Ar/CO2 | Protection of the weld pool from oxidation |
| Mode | Dotted or intermittent | Reduces body thermal deformation |
After completing the welding work, the seam must be cleaned with a grinder flush with the plane of the body. If there are small shells left, they can be melted with a special copper solder or high-quality body sealant. It is important not to overheat the metal so as not to disturb the structure of the factory galvanizing around the seam.
What to do if the metal is burned through during welding?
If you do burn a hole, don't panic. Clean the edges of the burn, reduce the current on the machine and try welding in spots, applying metal from the edges to the center. Sometimes it helps to use a thinner wire (0.6 mm) or a copper pad on the back side.
Use of fiberglass and epoxy compounds
For areas where welding is impossible or undesirable, fiberglass technology is excellent. Fiberglass in combination with epoxy resin creates a reinforcing layer that is not subject to corrosion. This method is especially popular when repairing arches and sills.
First, a layer of resin is applied to the degreased surface, then a piece of fiberglass cut to shape is applied. The material is thoroughly impregnated with resin with a brush to expel air. The process is repeated 2-3 times, creating a multi-layer “pie”. After polymerization, the resulting surface is polished.
⚠️ Attention: Epoxy compounds are sensitive to temperature and humidity during application. Do not work in the cold or in direct sunlight, as this will disrupt the polymerization process.
The main advantage of the method is absolute tightness and the absence of electrochemical corrosion, since glass fiber does not conduct current. However, on large planes such a patch can crack due to vibration, so it requires careful reinforcement and correct geometry.
Finishing, priming and painting
After the geometry is restored, the alignment stage begins. For this purpose it is used automotive putty. It is applied in thin layers, each time waiting until it dries completely and sanding with an abrasive. The task is to create a perfectly flat surface that follows the factory contours.
The key is applying the primer. Acid primer (phosphating) is applied first to bare metal or putty to protect against corrosion. This is followed by a filling acrylic primer, which hides sanding marks and creates a base for the paint. Paint Apply only after the primer layer has completely dried.
- 🎨 Apply 2-3 layers of acrylic primer with interlayer drying.
- 🎨 Treat the primer with a fine abrasive (P400-P600) before painting.
- 🎨 Cover the surface with base enamel and varnish in 2-3 layers.
The final touch is polishing the transitions if part of the part has been painted. A high-quality paint and varnish coating is the main barrier that protects your work from an aggressive external environment. Don't skimp on varnish, choose products with UV filters for added protection.
The quality and durability of rust-through repairs depend 90% on the thoroughness of corrosion removal and the quality of degreasing before applying materials.
Frequently asked questions (FAQ)
Is it possible to repair through rust with putty only?
Absolutely not. The putty is hygroscopic, it absorbs moisture and does not have anti-corrosion properties. Once applied to a hole, it will quickly fall off, and rusting will continue at double the rate under the layer of material.
Do I need to prime the inside of the patch?
Yes, definitely. Access to the inside is often limited, so before installing the patch it must be coated with zinc-containing primer or anti-corrosive. This will prevent corrosion on the back side that cannot be repaired later.
How long does it take for epoxy resin to dry before sanding?
The polymerization time depends on the air temperature and the type of hardener. Usually at +20°C, primary hardening occurs after 40-60 minutes, but the material gains full strength and the ability to grind after 12-24 hours. Don't rush, it's better to wait longer.
What abrasive should I use to remove rust?
For initial rough cleaning, use flap wheels with grit P40-P80. To level transitions and prepare underground, move to P120-P180. Final sanding for painting is carried out with abrasives P240 and higher.