The discovery of through corrosion or mechanical damage on the threshold of a car is always an unpleasant surprise for the owner, which absolutely cannot be ignored. The threshold is an important strength element of the body, providing structural rigidity, and its destruction can lead to serious consequences in an accident. Moreover, a hole in the metal opens direct access to moisture and dirt into hidden cavities, starting an irreversible process of rotting, which over time can spread to the bottom and side members.
In this article we will look in detail at how to properly seal a hole in a threshold, what materials are best suited for this, and in which cases welding is no longer necessary. Timely repairs will extend the life of the body for many years and preserve the resale value of the car. We will look at both professional methods using a welding machine, and affordable “cold” restoration methods for those who do not have access to garage equipment.
Before taking active action, it is necessary to assess the scale of the disaster, since the chosen recovery technology will depend on the depth of damage to the metal. Sometimes a small external hole hides a huge cavity of rotted metal, requiring complete overcooking of the element. Deep corrosion often affects internal amplifiers that are not visible upon superficial examination, so diagnosis must be careful.
Diagnosis of damage and assessment of the scope of work
The first stage of any quality repair is thorough cleaning of the damaged area from dirt, bitumen mastics and loose rust. Use a wire brush, sandpaper, or a special drill bit to get down to the bare metal and see the actual boundaries of the hole. It often happens that the visible hole, the diameter of a coin, is just the tip of the iceberg, and underneath the metal is thinned to the state of foil over an area of several square centimeters.
Particular attention should be paid to the inside of the threshold, which can be checked through technological holes or by removing part of the interior trim. If probing shows that the inside is also rotten, then simply patching the outside will not have a long-term effect. In such cases, an integrated approach is required with the treatment of internal cavities with anti-corrosion compounds.
⚠️ Attention: If, when tapped with a hammer, the metal rings like a tambourine or is easily pierced with an awl, simple putty will not help here - a metal patch or complete replacement of the element is necessary.
To accurately assess the condition of the metal, you can use a magnet wrapped in a thin cloth: it will help determine the thickness of the remaining metal and the limits of corrosion. After cleaning, be sure to degrease the surface to see the true picture of the damage and plan further actions. Only after a complete diagnosis can a decision be made on the repair method.
Preparing tools and materials for repairs
The success of the operation to restore the threshold directly depends on the quality of the materials used and the availability of the necessary tools. If you plan to use the welding method, you will need a semi-automatic gas (MIG/MAG) welding machine, as it produces less overheating of the metal than conventional arc welding. The “cold” method will require epoxy compounds, reinforcing meshes and a special tool for cleaning surfaces.
Regardless of the method chosen, you will definitely need personal protective equipment: goggles, a respirator and gloves, since working with rust and chemicals is harmful to health. Also prepare a set of sandpaper of different grits, degreaser, rust converter and primer. High quality surface preparation accounts for up to 80% of the success of the entire repair, so saving on consumables is not worth it.
The table below shows the main materials needed for different stages of work so that you can check their availability in advance:
| Material | Purpose | Features of choice |
|---|---|---|
| Epoxy glue/resin | Sealing and gluing | Two-component, with high adhesion to metal |
| Fiberglass/mesh | Patch reinforcement | Fine mesh, resin resistant |
| Zinc-containing soil | Anti-corrosion protection | Mandatory presence of zinc in the composition |
| Metal patch | Geometry restoration | Thickness 0.8-1.2 mm, steel without corrosion |
If you use welding, do not forget about the copper backing, which will help avoid burning through the thin metal during welding. An angle grinder with a thin cutting disc or special metal scissors are ideal for cutting rusty edges. Correctly selected tools will allow you to complete the work accurately and quickly.
Use a magnet with a string attached to prevent small metal chips from falling inside the threshold or into the interior of the car while working.
Cold welding method: step-by-step instructions
The cold welding method is ideal for repairing small through holes where using a welder is not possible or practical. This method is based on creating a durable composite layer of epoxy resin and reinforcing material that seals the damage. The main advantage of the method is the absence of thermal effects on the body, which eliminates the risk of metal deformation and burnout of the factory zinc coating around the repair area.
The process begins with carefully cleaning the edge of the hole, which must be given a conical shape for better adhesion of the material. The first layer of epoxy glue is applied to the cleaned and degreased metal, after which a piece of fiberglass or metal mesh cut to shape is applied. Reinforcing layer should overlap the edges of the hole by at least 2-3 centimeters on all sides for reliable fixation.
☑️ Cold welding algorithm
After the first layer has dried, a second, thicker layer of the mixture is applied, which forms the final geometry of the surface. It is important to work quickly as epoxy compounds have a limited working life and excess material will be difficult to remove later. To achieve ideal smoothness, you can use wet grinding after complete polymerization of the composition.
⚠️ Attention: The ambient temperature when working with epoxy resins must not be lower than +15°C, otherwise the polymerization reaction may not complete and the patch will remain sticky.
Cold welding is an excellent temporary or even permanent solution for non-power elements, if the technology is followed. However, it is worth remembering that such repairs do not restore the original rigidity of the body, but only seal the hole. For cars that are being prepared for sale or operation in gentle conditions, this is often quite sufficient.
Cooking thresholds: a professional approach
If the hole in the threshold is large or the metal around it is severely weakened by corrosion, the only correct solution is to install a metal patch using welding. This method allows you to restore not only tightness, but also power structure body, which is critical for passenger safety. For work, it is best to use semi-automatic welding in a shielding gas environment, as it provides a high-quality seam on thin body metal.
Before starting welding work, it is necessary to cut out all rotten areas of the metal to healthy tissue, giving the hole the correct geometric shape. A patch is cut from new sheet metal that is slightly larger than the hole to provide a seal. The edges of the patch and the body are carefully cleaned to a shine, after which you can begin tack welding the element.
Secrets of high-quality welding of thin metal
To prevent burns, use an interrupted weld, pausing briefly between tack welds to allow the metal to cool. You can also use a copper plate as a heat sink on the back side of the seam.
Cooking should be done in short seams (1-2 cm each) in a checkerboard pattern to avoid overheating of the metal and warping of the threshold. After each pass, let the metal cool by tapping the seam with a hammer to relieve stress. Welding seam must be continuous and sealed, without fistulas or lack of penetration, otherwise moisture will again begin to destroy the body from the inside.
After completing the welding work, the seam must be cleaned with a grinder or file, removing all irregularities and sagging. Then the surface is treated with a rust converter (if there has been heat treatment) and covered with zinc-containing primer. Only after this can you start filling and painting, being confident in the reliability of the connection.
Use of ready-made repair kits and aluminum tapes
The modern automotive chemical market offers many ready-made solutions for quick repair of thresholds, such as bitumen-based aluminum tapes or special repair kits. Aluminum tape with an adhesive layer is an express method that allows you to temporarily close the hole and protect the interior from water and dirt. It adheres well to degreased metal and can serve for several seasons if it is not subjected to strong mechanical stress.
Repair kits often include two-part fiberglass fillers that, when mixed, form a very hard and durable material. Such compounds are ideal for filling uneven holes and restoring the shape of the threshold before painting. Fiberglass putty does not absorb moisture and has excellent adhesion to metal, which makes it popular among car enthusiasts.
When using ready-made kits, it is important to strictly follow the manufacturer's instructions, especially regarding mixing time and drying time. Violation of the proportions can lead to the fact that the material does not harden or, on the contrary, becomes too brittle. To achieve the best result, it is recommended to combine these materials: first put a metal or aluminum patch, and level the surface with putty on top.
⚠️ Attention: Aluminum tapes and bitumen materials are not strength elements and do not protect against corrosion underneath - the metal underneath can continue to rot if high-quality anti-corrosion is not carried out.
These methods are good as a temporary solution or for preparing a car for sale when you need to fix a defect quickly and cheaply. However, for yourself and long-term use, it is better to choose more capital restoration methods that guarantee the durability of the result.
Finishing and anti-corrosion protection
After the hole is sealed and the surface is leveled, the most important stage begins - finishing and corrosion protection. Even the best quality repairs will not last long without the right anti-corrosion coating, since the threshold is constantly exposed to moisture, reagents and stone impacts. First of all, an acid or epoxy primer is applied to the repaired area, which provides adhesion of the paint to the metal and additional protection.
After the primer has dried, the surface is puttied, sanded and painted in body color or with protective paint for sills. Particular attention should be paid to the edges of the repair area where the new metal or patch meets the old body - this is where new corrosion most often begins. For painting, you can use aerosol cans of a selected color or paint in a can, applying it in several thin layers.
The main secret to the long life of thresholds is not so much the patch itself, but the high-quality anti-corrosion treatment of hidden cavities after repair.
The final step should be the treatment of the internal cavities of the threshold with special anti-corrosion compounds (Movil, cannon lard or modern wax compounds). To do this, additional holes are drilled in the threshold or existing technological ones are used, through which the protective composition is sprayed. Hidden cavities must be covered with a uniform layer of protection, which will displace any remaining moisture and preserve the metal.
Regular inspection and updating of the outer layer of protection (for example, applying anti-gravel) will allow you to forget about the problem of rusty thresholds for many years. Don't forget that even a small scratch down to the metal in the repaired area should be painted over immediately to prevent the rusting process from resuming.
Frequently asked questions (FAQ)
Is it possible to repair a hole in the threshold without welding at all?
Yes, reinforced epoxy or aluminum tape can be used for small holes and non-force areas. However, such repairs are considered temporary or semi-permanent as they do not restore body rigidity and require regular monitoring.
Do I need to remove the threshold from the car for quality repairs?
In most cases, it is not necessary to remove the threshold; it is enough to provide convenient access to the damaged area. Removing the threshold may only be necessary if the element is completely replaced or if corrosion has affected internal reinforcements that cannot be reached through technological openings.
What putty is best to use to repair thresholds?
It is best to use fiberglass putty for the initial filling of irregularities and holes, as it is durable and resistant to moisture. For finishing leveling, ordinary polyester putty is used, which is easier to sand and gives a smooth surface.
Why does the threshold rot even after repair?
Most often, the reason lies in poor surface preparation before repair (rust remains) or lack of protection for internal cavities. Moisture enters through microcracks in the paint or from below, and if the inside is not treated with anticorrosive, rotting continues from the inside out.