Through holes on the sills, arches or bottom do not appear overnight, but become the final stage of metal destruction, when surface rust turns into deep erosion. If you find that when pressed with a screwdriver, the metal falls through or light is already visible in the body, then surface treatment with anti-corrosive agent will no longer help here - a complete restoration of the geometry of the part is required using welding or epoxy composites. Ignoring such defects leads to the rapid growth of corrosion under adjacent body elements, which in a couple of months may require replacing the entire panel instead of local repairs.
The process of eliminating through corrosion is fundamentally different from the fight against “saffron caps” on the surface, since it requires restoring the load-bearing capacity of the area or at least the tightness of the body. Depending on the location of the hole and the accessibility of the reverse side, different techniques are used: from installing patches of new metal on the weld to using fiberglass mats for complex geometric shapes. It is important to understand that you cannot simply cover the hole with putty - it does not adhere to rust and does not hold the load, so preparing the edges and removing oxides are critical steps that determine the durability of the repair.
Diagnosis of the extent of damage and choice of method
Before starting work, it is necessary to accurately determine the boundaries of the damaged area, since the visible hole is often just the tip of the iceberg. Rust under paint can extend well beyond visible edges, creating hidden cavities. For an accurate diagnosis, use a sharp awl or screwdriver: lightly prick the metal around the hole. If the metal crumbles or is easily pierced, it must be stripped until clean, ringing metal appears, retreating from the visible edge of the rust by at least 2-3 centimeters.
The choice of restoration technology directly depends on whether the damaged part is load-bearing. For elements of the load-bearing frame, such as side members, sill reinforcements and rack mountings, the only acceptable option remains metal patchinstalled in compliance with welding technology. The use of fiberglass or putty on power elements is prohibited, since these materials cannot withstand vibration and shock loads, which can lead to deformation of the body during operation.
- 🔍 Visual inspection on both sides of the part to assess the through nature of corrosion.
- 🛠 Mechanical testing of metal hardness with an awl along the perimeter of the damage.
- 📏 Measuring the thickness of the remaining metal to understand whether it will withstand welding without burning.
- 🚗 Assessing the functionality of a part: whether it carries a load or is a decorative element.
⚠️ Attention: If rust has covered more than 40-50% of the area of the load-bearing part (for example, a threshold or spar), local repair may not be practical. In such cases, it is safer and cheaper to replace the entire element or weld in a new repair insert than to try to patch the sieve.
For non-strength elements such as exterior door panels, fenders or floor areas under carpets, composite materials are acceptable. However, even here careful surface preparation is required. It is important to remove all loose rust, as any remaining oxide will continue to attack the metal underneath the patch. After cleaning, the edges of the holes should be smooth, without torn edges, which will ensure a good fit of the repair material.
Surface preparation and corrosion removal
The quality of surface preparation determines 90% of the success of all repairs. If you skip the full stripping step, the new patch or layer of fiberglass will fall off within a few months along with chunks of rusty metal. First you need to remove the paintwork around the hole at a distance of 10-15 centimeters. To do this, it is convenient to use a grinding machine with a P80 or P120 abrasive wheel, or chemical paint removers if mechanical impact is limited.
After removing the paint, it’s time to remove the oxides. Use a metal brush (hand or in the form of a drill attachment) and a rust converter if the surface has a complex texture. However, for through holes, mechanical cleaning is mandatory. The edges of the hole must be processed to bare metal, chamfering at an angle of 45 degrees. This is necessary to increase the contact area of the repair compound with the body and create reliable mechanical engagement.
Degreasing is the final stage of preparation, which is often ignored, making a fatal mistake. There are always traces of oils, silicone or dust on the metal, which reduce adhesion. Wipe the prepared area White spirit or a special degreaser. Do not use gasoline or solvents with a high oil content. The surface must be completely dry and clean before applying any materials.
- 🧹 Removing the old paintwork and soil around the damage with a margin of 10-15 cm.
- ⚙️ Cleaning the metal to a shine, removing all pockets of corrosion.
- 📐 Beveling the edges of the hole for better adhesion of materials.
- 🧪 Thorough degreasing of the work area before starting restoration.
Metal patch welding technology
Welding is the most reliable way to close a hole in the body, ensuring the integrity of the structure. For thin body metal with a thickness of 0.6-0.8 mm, it is best suited semi-automatic welding (MIG/MAG) in a protective gas environment. Electric welding (MMA) is also applicable, but requires high skill so as not to burn through the metal and damage the anti-corrosion coating on the back of the panel.
The technology for installing a patch begins with cutting out a piece of metal. It should be the same thickness as the part being repaired, or slightly thinner. The shape of the patch should follow the contours of the hole with a small margin. First, the patch is tacked at several points around the perimeter to avoid warping of the metal from thermal stress. Then the continuous seam is welded in short sections (burns) in a checkerboard pattern, allowing the metal to cool between passes.
⚠️ Attention: When welding, be sure to use heat-dissipating copper plates on the back side of the seam. This will prevent burning of the metal and maintain the geometry of the part. Overheating can lead to burnout of the zinc layer and the factory anticorrosive, which will trigger a new wave of corrosion around the seam.
After cooling, the seam must be cleaned with a flap wheel or a file, comparing it with the plane of the body. High welding points are knocked down, and the recesses are filled with copper-containing putty, which dissipates heat better and is less susceptible to thermal expansion. It is important not to overheat the metal, as this changes its structure and makes the weld area brittle.
Welding wire selection
For body work, it is optimal to use wire with a diameter of 0.6 mm or 0.8 mm. Thicker wire (1.0 mm or higher) will create excess penetration and can burn through thin metal. The argon-CO2 gas mixture provides a more stable arc and less spatter than pure carbon dioxide.
Repairing through holes with fiberglass
The fiberglass and epoxy resin method is ideal for complex geometries where it is impossible to get to the back to install a metal patch, or for non-strength elements. This method allows you to create a durable, sealed and elastic layer that compensates for body vibrations. The basis is epoxy resin with a hardener, reinforced with layers of fiberglass or glass mat.
The process begins by applying a base coat of resin to the prepared hole edges and adjacent surface. Then the first layer of fiberglass is applied, which is thoroughly impregnated with resin using a brush or roller. It is important to squeeze out all air bubbles, as they are sites of future corrosion and weakening of the structure. After the first layer has polymerized (usually 20-40 minutes), the procedure is repeated 2-3 more times, each time using a larger piece of fabric to create a smooth transition.
| Material | Strength | Elasticity | Difficulty of application | Application |
|---|---|---|---|---|
| Fiberglass | High | Low | Average | Flat surfaces |
| Fiberglass | Average | High | Low | Complex shapes |
| Fiberglass (cut) | High | Average | High | Large volume filling |
| Carbon | Very high | Low | High | Tuning, not for repair |
After the composite layer has completely dried (usually 24 hours at a temperature of +20°C), the surface is sanded with coarse abrasive to remove irregularities and protruding fibers. The resulting “patch” may be stronger than metal, but it will not rust. However, if the edges are not properly cleaned, rust can creep out from under the fiberglass, so the quality of edge preparation is critical here.
- 🧴 Applying a primer layer of epoxy resin to metal.
- layers Layer-by-layer laying of fiberglass with thorough impregnation.
- 💨 Removing air bubbles for a solid structure.
- 🪞 Final sanding of hardened fiberglass for painting.
Finishing and anti-corrosion protection
After the hole is sealed by welding or fiberglass, the final preparation stage for painting begins. If welding was used, the metal in the weld area is left unprotected and requires immediate coating. Best suited for primary protection epoxy primer, which creates a barrier to moisture and oxygen. Acid primers (phosphate) are also effective, but they require obligatory covering with acrylic primer, since they themselves are hygroscopic.
Automotive putty is used to level the surface. For through-hole repairs, it is recommended to apply fiberglass putty as the first layer. It has less shrinkage and greater strength than all-purpose polyester putty. It should be applied in thin layers, waiting until each layer is completely polymerized before applying the next. The layer thickness should not exceed that recommended by the manufacturer to avoid peeling.
☑️ Checklist before painting
The final stage is the application of anti-corrosion compounds to the internal cavities and the reverse side of the repair. If access is possible, use wax or oil-based anticorrosion agents with a long spray nozzle. They penetrate microcracks and displace remaining moisture, creating an elastic film. On the outside, under the paint, this function is performed by a high-quality enamel primer.
⚠️ Attention: Never paint over rust or putty without first priming. The paint allows moisture to pass through (vapor permeable), and without a soil barrier, the metal underneath will rust in one season. The soil is the only real protection for the body.
Common mistakes when doing DIY repairs
Self-repair of body damage is full of nuances, failure to comply with which reduces all efforts to zero. One of the most common mistakes is trying to fill a hole with just polyester putty. This material is not structural: it is afraid of moisture, shrinks, and during vibrations it simply cracks and falls out. The putty is intended only for leveling the microrelief, but not for restoring geometry or tightness.
Another mistake is not cleaning the edges enough. Many beginners only remove visible rust, leaving a transition zone where the metal is already weakened. After six months, active corrosion begins under the new patch, and the hole opens again, often becoming larger. It is also dangerous to neglect the anticorrosive agent on the reverse side: a hole welded from the outside continues to rot from the inside, where moisture has reached.
Use a magnet wrapped in cotton wool on a wire to check for metal under the putty after repairs. If the magnet does not stick in some place, then there is a cavity or a thick layer of unreinforced material left there.
Using inappropriate materials, such as foam or caulk, to fill large holes before filling is also a mistake. Foam is hygroscopic and accumulates moisture, turning the repair into a source of rot. If you need to fill the volume, use special fiberglass fillers or cut precise metal inserts.
The main secret to the longevity of the repair is not in expensive materials, but in the thoroughness of stripping rust to bare metal and high-quality anti-corrosion treatment on the back side of the part.
Questions and answers
Is it possible to repair a hole in the body using cold welding?
Cold welding (metal-filled epoxy adhesive) is only suitable for temporary repair of small fistulas or cracks in inappropriate areas. It cannot withstand strong vibrations and shock loads. For permanent repairs of through holes, it is better to use fiberglass or metal welding.
Does fiberglass need to be primed before puttying?
Yes, definitely. Fiberglass has a porous structure and can absorb styrene from the putty, which will disrupt its polymerization. In addition, the soil evens out absorption and improves adhesion. Use epoxy or acrylic primer.
How long does fiberglass epoxy take to dry?
The polymerization time depends on the air temperature and the amount of added hardener. At +20°C, primary curing occurs after 40-60 minutes, but the material gains full strength and chemical resistance after 24 hours. Do not speed up the process with a hairdryer, as this may cause the resin to boil.
What is better to seal holes in thresholds: with boiling or glass?
Thresholds are a strength element of the body. According to safety and technology rules, they need to be restored only by welding metal. The fiberglass on the rocker panels can crack when the body is subjected to stress (such as being jacked up or falling into a hole), causing the body to lose rigidity.