Damage to sills is one of the most common problems that car owners face while operating their vehicle. Gravel, curbs, sloppy parking lots and aggressive chemicals on the roads leave their mark, turning a once smooth part into a source of corrosion and an aesthetic defect. That is why the question of how to level the threshold of a car becomes relevant for many drivers who want to return their car to its original appearance.
Repairing this area of the body requires a special approach, since the thresholds often perform not only a decorative, but also a structural function, increasing the rigidity of the body. Ignoring damage can lead to serious consequences, including disruption of the body geometry and the spread of rust on the bottom. In this article we will analyze in detail all stages of restoration, from diagnosis to final painting.
Diagnosis of damage and choice of restoration method
Before taking up tools, it is necessary to objectively assess the scale of the disaster. A visual inspection is only the first step and often does not provide a complete picture of the condition of the metal. Rinse the damaged area thoroughly with water and disinfectant to remove dirt and bitumen that hide the true depth of the defects.
There are two main types of damage: dents without breaking the metal and through corrosion or tears. In the first case, you can get by by pulling and straightening, the second will require a more complex procedure with cutting out the repair kit. If, when pressing on the edge of the threshold, the metal is dented or a crunch is heard, it means that irreversible processes of destruction have begun.
To accurately assess the thickness of the paintwork and identify hidden areas of corrosion, professionals use a thickness gauge. However, in garage conditions, you can use a regular magnet wrapped in thin fabric: in putty areas the force of attraction will be noticeably weaker.
- 🚗 Surface dents - deformation without damage to the paintwork, amenable to drawing.
- 🔨 Deep creases — violation of the metal structure, requiring straightening.
- 🕳️ Through corrosion - complete destruction of the metal, an insert is needed.
- 🎨 Damage to paintwork - scratches and chips requiring local painting.
By determining the type of damage, you can choose the right repair strategy and purchase the necessary materials without overpaying for unnecessary consumables.
Necessary tools and materials for body work
The quality of repairs directly depends on the preparation of the workplace and the availability of specialized equipment. To fully restore thresholds, you will need not only a hammer and sandpaper, but also a more serious arsenal. The basic set includes a grinder with a set of cutting discs, a welding machine (preferably semi-automatic), a drill and a grinder.
Particular attention should be paid to personal protective equipment. When working with metal and chemicals, be sure to use a respirator, safety glasses and gloves. Dust from old paint and solvent fumes can cause serious health damage.
⚠️ Attention: Never carry out welding work near a fuel tank or gas line without first dismantling or properly insulating the fuel system. A spark may cause a fire.
The list of consumables is also long. You will need:
- 🛡️ Anti-corrosion primers to protect metal.
- 🧴 Degreaser for surface preparation.
- 🧱 Fiberglass putty for initial alignment.
- 🎨 Epoxy primer for insulating metal from moisture.
Do not skimp on materials, since the thresholds are in constant contact with water, dirt and reagents. Cheap putty may fall off after a couple of months, ruining all your work.
Paintless Dent Repair (PDR) technology
If the paintwork is intact and has no cracks, it makes sense to try the PDR (Paintless Dent Repair) method. This technology allows you to level the sill while preserving the factory paint, which is especially valuable for cars with complex colors such as mother of pearl or metallic.
The essence of the method is to influence the metal from the inside or use special hooks and suction cups from the outside. To access the inside of the threshold, it is often necessary to dismantle the seats, remove the thresholds, or drill technological holes in hidden places, which are then closed with plugs.
The process requires patience and good lighting. Using a light panel, the master sees the slightest reflection distortion on the body and carefully squeezes out the dent. It is important not to overdo it, since metal after deformation has a “memory” and may tend to return to its original state or, conversely, burst from overstrain.
If the dent is on a stiffener or has a complex shape, the PDR method may not give an ideal result. In such cases, you have to resort to classic straightening followed by touch-up.
Is it possible to heat metal when leveling?
Using a hair dryer is only permissible on certain types of steel and requires caution. Overheating can cause the metal to loosen, lose its strength and cause the paint to swell. It is better to use mechanical drawing methods.
Welding work and replacement of threshold sections
In cases where corrosion has eaten away the metal or the dent is too deep, the only solution is to replace part of the threshold. For this, a special repair profile is used, which can be purchased at auto parts stores. It can be solid or composite.
The first step is to cut out the damaged area. This must be done with reserve, removing all rusty metal to a clean, healthy base. The edges of the cut must be smooth so that the new part fits like a glove. If you are changing part of the outer threshold, the inner part (the amplifier) also needs to be checked and, if necessary, restored.
Welding of a new part is carried out by spot welding or a continuous seam in a protective gas environment. It is important not to overheat the metal by making short welds with breaks for cooling. This will prevent body deformation from thermal expansion.
| Type of work | Tool | Difficulty | Lead time |
|---|---|---|---|
| Fine straightening | Hammer, anvil | Average | 2-4 hours |
| Replacing part of the threshold | Welding, grinder | High | 6-10 hours |
| Complete threshold replacement | Welding, slipway | Very high | 1-2 days |
After welding, the seams must be cleaned with a flap disc so that they do not protrude above the surface of the body. A perfectly flat surface is the key to ensuring that the putty will lie flat and not crack.
☑️ Preparation for welding
Puttying and preparing the surface for painting
Even after high-quality straightening or welding, the surface of the threshold is far from ideal. Micro-irregularities, risks from the grinder and height differences remain on it. This is where automotive putty comes into play. The first layer is always putty with fiberglass, which provides strength and fills large holes.
After the first layer has dried (usually 20-30 minutes at room temperature), it is sanded with coarse sandpaper (P80-P120 grit). Then a finishing putty is applied, which has a finer fraction and is easier to process. It removes small pores and scratches.
It is important to follow the application technology: the putty must be applied with sharp, confident movements of the spatula at an angle of 45-60 degrees, pressing it tightly to the surface. You should not try to level everything in one centimeter thick layer - it will definitely crack.
Use developing powder or dark colored spray paint before sanding the putty. This will allow you to visually see which areas still need to be processed, and which have already reached the plane.
Final sanding is carried out with sandpaper P240-P320 under the primer. The surface should be smooth to the touch, without steps or transitions between the metal and the repair area.
Priming, painting and corrosion protection
The final stage is the creation of a protective and decorative coating. After degreasing the surface, an epoxy primer is applied, which creates an airtight film that prevents oxygen and moisture from accessing the metal. This is a critical step for the longevity of the repair.
This is followed by an acrylic filler primer, which hides sanding marks and creates a base for the paint. After the primer has dried (usually 12-24 hours), the surface is sanded again with a fine P500-P600 abrasive. Now you can start painting.
The paint is applied in 2-3 layers with interlayer drying for 10-15 minutes. It is important to maintain the temperature and humidity conditions in the spray booth. After the paint has dried, varnish is applied (2 layers), which adds depth to the color and protects the paint from ultraviolet radiation and mechanical damage.
⚠️ Attention: Before applying varnish, make sure that the base is completely dry. If you apply varnish to half-dry paint, dull spots or “apple-coloring” may appear after a while.
For additional protection, the lower part of the threshold can be treated with an anti-gravel compound. This will give the surface texture and protect it from stone impacts.
The quality of painting depends 80% on proper surface preparation. Don't be lazy during the sanding and degreasing stages.
Frequently asked questions (FAQ)
How long does putty dry before painting?
Drying time depends on air temperature and the amount of hardener added. At +20°C, standard polyester putty dries in about 20-30 minutes. However, before applying the primer, it is better to let it stand for at least an hour for the styrene to completely evaporate.
Do the thresholds need to be removed for repairs?
In most cases, the sills are welded to the body and cannot be removed. You only need to remove the plastic lining (if any) and interior elements for access from the inside. Dismantling the metal threshold itself is possible only if it is completely replaced with cutting.
What is the best way to weld a hole in a threshold?
The best option is to insert a repair pad made of steel of the same thickness as the body. Tin sealing or using mesh epoxy are temporary solutions and do not provide long-term corrosion protection.
Which putty should I choose for the threshold?
For thresholds subject to vibration, putty with aluminum powder or reinforced with fiberglass is ideal. They are more flexible and impact resistant than conventional finishing compounds.