Body parts hidden from the driver's eyes often become the source of serious problems if attention is not paid to them in time. Replacing thresholds - this is one of the most difficult bodywork operations, requiring not only special tools, but also a deep understanding of the geometry of the car. Ignoring the first signs of corrosion can lead to the fact that the strength structure of the body will be damaged, and restoration will become economically unfeasible.
Owners are often faced with a dilemma: trying to digest old elements or install new ones. In modern conditions, when requirements for safety and environmental friendliness are growing, threshold repair Without replacement, it often gives only a temporary effect. Rot inside hidden cavities spreads faster than rust in visible areas, making visual assessment of body condition unreliable.
In this article we will analyze in detail the entire technological process, from diagnostics to anti-corrosion treatment. You will learn what tools are needed for quality work, how to properly prepare the body for welding and why spot welding is considered a more reliable joining method than semi-automatic when working with thin sheet metal.
Diagnostics and selection of repair method
Before proceeding with the purchase of materials, it is necessary to objectively assess the scale of the disaster. Often visual attractiveness overhead thresholds hides a completely rotted internal structure. For an accurate diagnosis, you should carefully open the accessible areas or use an endoscope, if possible. If the metal crumbles from lightly pressing a screwdriver, then partial repairs no longer make sense.
There are two main ways to solve the problem: completely replacing the element or installing repair inserts. A complete replacement involves dismantling the entire outer threshold and part of the inner spar beam. This method is the most labor-intensive, but guarantees long-lasting results. The insert is used when the central part is intact, and only the edges of the arches or pillars are rotting.
⚠️ Attention: If corrosion has affected the seat belt attachment points or jack installation points, operation of the vehicle is prohibited until the power structure is completely restored!
When choosing between the welding method and installing linings, it is important to consider the purpose of the vehicle. For daily use in harsh conditions, welding is preferable, since moisture will still accumulate under the pads, continuing to destroy the metal. However, if the goal is to quickly prepare the car for sale or hide defects in non-critical areas, decorative overlays may be a temporary solution.
Required tools and materials
The quality of the work performed directly depends on the equipment used. To professionally replace thresholds, mere desire is not enough; you will need a serious set of tools. The main unit will be a welding machine: a semi-automatic machine (MIG/MAG) with wire feeding in a protective gas environment is best, although experienced craftsmen often use argon or spot welding to minimize thermal deformation.
In addition to welding, you will need an angle grinder (grinder) with a set of cutting and grinding wheels. It is important to have a drill with metal drill bits and possibly an air chisel on hand to quickly remove old weld spots. Don't forget about clamps of various sizes that you will use to secure new parts while tacking.
- 🛠️ Welding machine (semi-automatic or inverter for argon)
- 🔪 Angle grinder (grinder) with consumables
- 📏 Tape measure, marker and scriber for marking
- 🧤 Personal protective equipment (chameleon mask, leggings)
- 🧪 Primers, putty and anti-corrosion compounds
Pay special attention to consumables. The metal for the inserts should be the same thickness as the factory body sheet, usually 0.8–1.0 mm. Using thicker metal will lead to difficulties during welding and the risk of burning through, while thinner metal will not provide the necessary rigidity. Shielding gas (a mixture of argon and carbon dioxide) is required to obtain a high-quality seam without pores.
Use a copper plate when welding in hard-to-reach places - it serves as a heat sink and prevents burning through the metal.
Body dismantling and preparation
The process of removing the old threshold begins with dismantling all interfering elements: plastic trims, moldings, door seals and, in some cases, seats and trim to gain access to internal mounting points. After this, a neat cut of the rotten metal is made. The main rule is not to damage the inside of the spar, if it is preserved.
Cleaning the edges is a critical step. All cuts must be cleaned to a shiny metal to ensure a perfect weld seal. If rust remains on the internal surfaces, it must be treated with a converter, but it is better to mechanically remove the entire oxidized layer. This will ensure the durability of the repair.
⚠️ Attention: When working with an angle grinder, use a respirator and safety glasses. Metal dust and sparks can cause serious health and eye damage.
After mechanical cleaning, the surfaces are degreased. To do this, use a special degreaser or white spirit. Do not use gasoline or solvents that leave a greasy film, as this will lead to defects during painting and welding. The surface must be absolutely clean, dry and matte.
Technology for setting a new threshold
Installation of a new element begins with fitting. The part must perfectly follow the contours of the body, the gaps between the threshold and the door must be uniform along the entire length. If the geometry is broken, it is necessary to use a slipway or hydraulic braces to restore the correct dimensions of the opening. Without this, the door may not close or, conversely, hang loose.
The part is fixed with clamps. First, the element is tacked by welding at several points, then the geometry is checked again. If everything is in order, a continuous seam or a series of spot welds are made in increments of 2-3 cm. It is important not to overheat the metal, allowing it to cool in order to avoid “lead” - wave-like deformation of a thin sheet.
| Parameter | Meaning/Description | Impact on the result |
|---|---|---|
| Metal thickness | 0.8 – 1.0 mm | Matching factory hardness |
| Wire diameter | 0.8 mm | Optimal for body work |
| Current strength | 60 – 90 A | Depends on the thickness of the metal |
| Gas consumption | 6 – 8 l/min | Weld pool protection |
After welding, the seams are cleaned with a flap wheel to the level of the base metal. This is necessary to apply putty and obtain a smooth surface. If you use the insertion method, the joints are also carefully welded and sanded. Sealing internal cavities are carried out before final assembly so that moisture does not get inside the structure.
☑️ Welding quality control
Putty, primer and painting
After welding work, the surface is rarely perfectly smooth, so the application of putty is required. Epoxy or polyester compounds intended for bodywork should be used. The putty is applied in a thin layer and, after drying, sanded with an abrasive grade from P80 to P240 to create a smooth transition.
Priming is a step that should not be skipped. An acid (phosphate) primer is applied to the bare metal for chemical protection, then an acrylic filler is applied to smooth out micro-irregularities. Each layer must dry strictly according to the manufacturer's instructions. Acrylic primer also serves as an insulator between the metal and the finish coating.
Painting is carried out in a painting booth or in a clean, dust-free room. The paint is applied in 2-3 layers with interlayer drying. After the base enamel has dried, varnish is applied (usually 2 layers). The varnish protects the color from fading and adds depth. After complete polymerization, the surface is polished to remove shagreen and dust particles.
Secrets of perfect painting
To obtain the factory “orange peel” (shagreen) effect, it is important to correctly set the spray gun spray pattern and pressure. Too much pressure will break the paint into a mist, creating a dry, rough finish, while too little pressure will cause greasy streaks. The optimal distance from the nozzle to the surface is 15-20 cm.
Anti-corrosion treatment and protection
Even the highest quality painting will not protect the thresholds from the inside, where moisture, salt and dirt accumulate. Therefore, the final, but most important stage is the anti-corrosion treatment of hidden cavities. For this purpose, special wax or oily compounds are used, which are introduced into the threshold through technological holes using a long spray nozzle.
The composition must have penetrating ability, displace water and create an elastic film that does not crack when the body vibrates. Treatment should be carried out not only on new thresholds, but also on the entire bottom of the car, especially if the car is several years old. This will extend the life of the body for many years.
⚠️ Attention: Do not use used motor oils for anticorrosion! They contain aggressive additives and acids that can accelerate corrosion from the inside, and also have an unpleasant odor that penetrates the interior.
Regular maintenance is the key to success. Once every 1-2 years it is recommended to renew the anti-corrosion coating, especially after winter use. Visually inspecting the drainage holes will also help prevent water from pooling inside the sills. Timely prevention It costs several times less than replacing the elements again.
High-quality anti-corrosion treatment of hidden cavities is more important than the thickness of the paint layer on the outside, since rotting begins from the inside.
Frequently asked questions (FAQ)
How long does it take to replace thresholds in a garage?
On average, if you have experience and tools, replacing one threshold takes from 1 to 2 business days. This includes dismantling, welding, filling and initial painting. Complete drying of the varnish and polishing may require another 24 hours.
Is it possible to change the thresholds without welding, using only glue?
The use of body glue is only permissible for decorative trims or in combination with mechanical fastening (rivets). Glue will not replace the load-bearing capacity of a weld, so this method is not recommended for load-bearing elements.
Do I need to remove the doors when replacing thresholds?
It is not necessary to remove the doors, but it is highly recommended. This will provide better access to the upper welding points and will avoid damage to the door paintwork from sparks or tools. In addition, it is easier to control the gaps.
What metal is best to use for inserts?
It is optimal to use cold-rolled steel with a thickness of 0.8–1.0 mm. Aluminum or stainless steel cannot be used due to the difference in electrochemical potentials (galvanic couple), which will lead to accelerated corrosion at the point of contact with the steel body.