Rust on the sills is not just a cosmetic defect, but a signal of serious structural problems of the body, which, if ignored, can lead to loss of geometry and safety. When the owner notices through corrosion, he is faced with a difficult choice: contact a specialized service or try to restore the integrity of the metal on his own, which requires certain equipment and skills. Deep pockets of destruction often hide behind exfoliated internal metal, which, when tapped, produces a characteristic dull sound, indicating delamination of the structure.
The process of restoring the tightness of power elements requires careful preparation, selection of the right technology and strict adherence to the sequence of operations, otherwise all efforts will be in vain. In this article we will analyze proven methods of combating corrosion, from classical argon welding to the use of modern composite materials, allowing to extend the life of the car without expensive intervention. It is important to understand that sealing a hole properly means not just closing the hole, but stopping the further spread of rust.
Diagnosis of condition and selection of repair technology
Before taking up the tools, it is necessary to objectively assess the scale of the disaster, since surface rust and a through hole require fundamentally different approaches to elimination. Often the external attractiveness of the paintwork is deceptive, and under a thin layer of paint there is completely rotten metal that crumbles at the slightest mechanical impact. For an accurate diagnosis, experienced craftsmen use a magnet wrapped in a thin cloth that does not stick to areas where the metal has turned into iron oxide.
If the defects occupy less than 30% of the threshold area, it makes sense to try to cut out the damaged area and weld a repair insert, preserving the main part of the structure. In cases of total destruction, when not only the external but also the internal amplifier has rotted, the only safe solution is to completely replace the element, since local repairs will not provide the necessary rigidity of the body. The choice of method directly depends on the grade of steel, the thickness of the metal and the availability of equipment for the work.
There are several main recovery methods, each of which has its own advantages and limitations on the time and cost of project implementation:
- ⚡ Semi-automatic welding - a classic method that provides maximum joint strength, but requires highly qualified performers.
- 🛡️ Application epoxy compounds with reinforcing mesh - suitable for small holes and a temporary solution that does not require welding equipment.
- 🔩 Installing repair patches is a quick way to close holes, often used before selling a car to visually improve its condition.
- 🧪Usage cold welding - an emergency measure for sealing small fistulas, not designed for long-term operation under load.
⚠️ Attention: Never seal through holes in thresholds with regular putty without first installing a metal base. The putty is hygroscopic, it absorbs moisture from the air, and after a few months, an active corrosion process will begin again under the paint layer, which will be much more difficult to stop.
Necessary tools and materials for work
The quality of the work performed directly depends on how correctly the tools and consumables are selected, savings on which can reduce all efforts to zero. For a complete repair, you will need not only an angle grinder and a welding machine, but also specialized chemicals to protect the metal from the reappearance of rust. It is important to prepare the workplace in advance, ensuring good ventilation and lighting, since working with the body requires attention to detail.
The main list of equipment includes an angle grinder with a set of cutting and grinding wheels that allow you to carefully cut out damaged areas and clean the edges to bare metal. If welding is planned, then it is necessary semi-automatic with gas protection (usually a mixture of argon and carbon dioxide is used) or high-quality electric welding, although the latter is more prone to burning through thin metal. Don’t forget about personal protective equipment: a welder’s mask, gloves and a respirator are mandatory safety attributes.
For anti-corrosion treatment and finishing you will need the following materials:
- 🧴 Orthophosphorus-based rust converter to neutralize residual corrosion in hard-to-reach places.
- 🎨 Primer-enamel with zinc, creating a barrier layer and ensuring adhesion of the finish coating to the metal.
- 🕸️ Fiberglass or metal mesh for reinforcing repair compounds when using chemical methods.
- 🔧 Degreaser (anti-silicone) for removing technical oils before applying any coatings.
Particular attention should be paid to the selection of welding wire, the diameter of which must correspond to the thickness of the body metal, usually 0.6 or 0.8 mm. Using a wire that is too thick will lead to overheating and deformation of thin-walled elements, while a wire that is too thin will not provide proper penetration. The optimal solution for thresholds is to use copper-plated wire with a diameter of 0.8 mm, which ensures stable arc burning and minimal spattering.
Surface preparation and corrosion removal
The preparatory stage is the most labor-intensive and dirty, but 90% of the success of the entire body restoration event depends on it. It is necessary to remove not only visible rust, but also all layers of old varnish, paint and primer, getting down to clean, shiny metal, otherwise the new repair layer will not stick. It is better to carry out work in dry weather or in a well-heated garage, since air humidity negatively affects the quality of welding and the polymerization of chemical compounds.
The cleaning process begins with the mechanical removal of loose masses of corrosion using a metal brush or an abrasive wheel on a grinder. After removing the bulk of the rust, the surface is thoroughly cleaned with a flap wheel or sandpaper with a grit of P80 to P120 to remove the oxide film and create risks for better adhesion. It is important not to overdo it and not to thin the metal to critical values, especially in areas where the metal still retains its structure.
If rust has penetrated into hidden cavities, it is necessary to drill technological holes in inconspicuous places to treat the internal surfaces with anti-corrosion agent. Through these holes, using a long spray nozzle, a rust converter and liquid anticorrosive are blown in, which creates a protective film on the inner walls of the threshold. After drying, the holes are carefully welded or closed with rubber plugs.
☑️ Threshold preparation checklist
After mechanical cleaning, the surface must be degreased, since even traces of sebum can lead to peeling of the coating. To do this, use a special degreaser, which is applied to a lint-free cloth, and the entire work area is wiped. You can't Use gasoline or solvents containing oils for degreasing, as they leave a greasy film.
Technology of welding and installation of repair inserts
Welding is the most reliable way to restore the integrity of the thresholds, allowing the element to return to its original strength. The essence of the method is to cut out the damaged area and install in its place a new part cut from sheet metal of the appropriate thickness. It is important to maintain a gap between the parts, which should be approximately 1-2 mm, to ensure high-quality penetration without the formation of burns.
The welding process begins with tacking the insert at several points around the perimeter, after which the compliance of the geometry and gaps is checked. Then a continuous seam is welded with intermittent movements to avoid overheating of the metal and its warping. After each 3-5 cm long section, it is necessary to allow the metal to cool or switch to the other side in order to distribute the thermal load evenly.
Comparison of the main welding methods for bodywork:
| Parameter | Semi-automatic (MIG/MAG) | Manual arc (MMA) | Argon (TIG) |
|---|---|---|---|
| Operation speed | High | Average | Low |
| Seam quality | good | Satisfactory | Excellent |
| Risk of burning | Minimum | High | Medium |
| Difficulty of mastering | Low | High | Very high |
After completing the welding work, the seam must be cleaned with a grinder and a grinding wheel, removing all irregularities and metal deposits. If the insert does not lie perfectly flat, it is possible to use a special body putty with fiberglass, which is capable of leveling out small differences in height. However, it should be remembered that the putty layer should not be too thick to avoid cracks due to body vibrations.
Secrets of an even seam
To obtain an even and strong seam when welding thin metal, use the “corner” technique. Hold the torch at a 45 degree angle to the plane of the part and move it from one edge to the other in short bursts. This allows the metal to melt and fill the gap without burning a hole through. It is also useful to use a copper backing on the back of the seam, which removes excess heat.
Alternative Methods: Epoxies and Mesh
In situations where a welding machine is not available or the welder's skills leave much to be desired, chemical restoration methods can be used. The essence of the technology is to create a reinforced layer of epoxy resin and fiberglass that seals the hole and prevents further destruction. This method does not restore structural strength, but is excellent for sealing and protecting against moisture.
To work, you will need two-component epoxy glue, a piece of fiberglass or fine-mesh metal mesh and a spatula. First, a base layer of resin is applied to the cleaned and degreased metal, then a reinforcing material is applied, which is carefully rolled with a roller to remove air bubbles. The process is repeated several times until the required thickness and strength of the patch is achieved.
Modern epoxy compounds have high adhesion to metal and resistance to vibration, which makes them popular among car enthusiasts. However, it is important to understand that such repairs are a temporary solution and require regular monitoring of the condition. Under strong mechanical loads or impacts from stones from under the wheels, such a patch may crack.
⚠️ Attention: When working with epoxy resins and hardeners, be sure to use rubber gloves and a respirator. Vapors from the components are toxic and may cause severe respiratory irritation or an allergic reaction. Work only in a well-ventilated area.
Helpful advice: To speed up the polymerization of the epoxy composition in the cold season, you can use a hair dryer. Warm up the patch with warm (not hot!) air, but make sure that the temperature does not exceed 60-70 degrees, otherwise the resin may boil and lose its properties.
Finishing and anti-corrosion protection
The final stage of repair is the application of protective coatings that will protect the restored area from aggressive environmental influences. Even the highest quality repairs are meaningless without competent anti-corrosion treatment, since moisture will inevitably get onto the thresholds during operation. The first layer is always an acidic primer or converting primer, which chemically bonds with the metal.
After the primer has dried, the surface is covered with an acrylic filler primer, which evens out minor defects and creates a base for paint. If the color of the car does not require precise selection (for example, the bottom of the threshold is being repaired), you can use universal enamel or even bitumen mastic, which has excellent moisture-repellent properties. It is important to paint over not only the outer, but also the inner part of the threshold through the technological holes.
For internal treatment of hidden cavities, wax or oily compounds are best suited, which do not dry out completely and remain mobile, filling all microcracks. Such compounds are applied using a long nozzle with a pressure sprayer, ensuring uniform coverage of all hard-to-reach corners. Regular updating of the internal anticorrosive (once every 2-3 years) significantly extends the life of the body.
High-quality repair of thresholds can save significant money and extend the life of the car for several years. The main thing is to take your time, follow technology and use proven materials, without relying on “folk” methods such as blowing foam or covering with plasticine. Durability repair depends on thorough preparation and protection.
High-quality anti-corrosion treatment of the internal cavities of the threshold is more important than perfect painting on the outside, since rotting begins from the inside.
Is it possible to weld a hole in the threshold without removing the element?
Theoretically it is possible, but it is highly not recommended. When welding on a car, there is a high risk of damaging the wiring, interior or other adjacent parts from sparks and heat. In addition, it is difficult to provide access to the inside of the seam for high-quality anticorrosive treatment. The removed threshold is easier to clean, boil on all sides and cover with protective compounds.
What putty is best to use for thresholds?
For thresholds that are subject to constant gravel impacts and vibrations, fiberglass putty is best suited. It has increased mechanical strength and elasticity. Regular soft putty should only be used for finishing leveling over the fiber layer.
Is it necessary to prime galvanized metal before welding?
The zinc coating burns out during welding, producing toxic fumes and deteriorating the quality of the weld. The welding site will still have to be cleaned to bare metal. However, it is advisable to treat the remaining surface of the galvanized part with a special primer for zinc surfaces in order to maintain its corrosion resistance.
How long does it take for the anticorrosive agent to dry inside the threshold?
Drying time depends on the type of composition and air temperature. Oily compositions can remain liquid for years, fulfilling their function. Wax and bitumen compounds usually polymerize from 24 to 48 hours at a temperature of +20°C. Solvents may take up to a week to completely evaporate.