Body corrosion is the main enemy of any car, especially in our climate, where roads are generously treated with reagents. The lower parts of the body are always the first to be hit, and the question of how to digest the sills sooner or later faces every owner of an older car. Rot in this area not only spoils the appearance, but also directly affects safety, since the threshold is an important structural element. If the destruction process is not stopped in time, rust will spread to the racks and bottom, turning repairs into a financially impractical undertaking.
Replacing thresholds yourself is a complex, but quite feasible project for a garage technician with welding and metal working skills. You don't have to be a professional bodybuilder to get the job done right, but you will need patience, the right tools, and the right technique. In this article, we will analyze in detail all the stages: from assessing the condition and purchasing materials to final painting and anti-corrosion treatment, so that you can extend the life of your car for many years.
Assessment of condition and preparation of tools
Before picking up an angle grinder, you need to soberly assess the scale of the disaster. Often the external condition of the paintwork is deceptive: swollen paint can hide a through hole, and sometimes only the outer apron looks rotten, while the inside is still intact. For an accurate diagnosis, use a magnet wrapped in a thin cloth, or simply gently tap suspicious areas - a dull sound will indicate peeling metal or putty. If the rust has covered more than 50% of the threshold area or affected the attachment points of seat belts and door hinges, then partial overcooking may not provide the required rigidity.
The quality of work performed directly depends on the availability of suitable equipment. You will need not just a welding machine, but also the right set of equipment for cutting and stripping. The optimal choice for body work is semi-automatic gas shielded welding (MIG/MAG), as it burns less through thin metal compared to an electrode. It is also critical to prepare the work area: the vehicle must be parked on level ground, and all flammable materials must be removed from the sparking area.
- 🛠️ Angle grinder (grinder) with a set of cutting and cleaning wheels of different thicknesses.
- ⚡ Semi-automatic welding machine with wire with a diameter of 0.8 mm and a cylinder with carbon dioxide or a mixture of gases.
- 🔨 A set of body hammers, files, chisels and clamps (clamps) for fixing parts.
- 🧴 Degreaser, primer, paint to match the body and materials for anti-corrosion treatment of hidden cavities.
⚠️ Attention: Before starting work, be sure to remove the negative terminal from the battery and turn off all electronic components if they are in the welding area to avoid voltage surges and damage to the electronics.
Don't forget about personal protective equipment. Welding produces not only bright radiation that is dangerous to the eyes, but also toxic smoke, especially if you are working with old metal coated with bitumen mastics. A respirator, gaiters and a high-quality welding shield are a mandatory minimum. It is also worth preparing a fire extinguisher and a bucket of water in case of an unexpected fire in the insulation or interior trim, which will have to be dismantled.
Selection of materials and dismantling of old thresholds
The most important step is choosing a repair insert. The market offers many options: from cheap stampings made of thin metal to high-quality galvanized analogues. It is best to look for original repair inserts for your car model, as they perfectly match the geometry. If it is impossible to find the original, you will have to select a universal profile or bend the part manually, which requires high qualifications. The metal thickness of the new insert must correspond to the factory one, usually 0.8–1.0 mm; Thicker metal is more difficult to weld, and thin metal will burn quickly.
Removing rotten elements requires care so as not to damage adjacent body parts, such as the floor, side members and racks. The doors must first be removed to allow easy access and to avoid damaging the hinges or door paint from sparks. Then, using a grinder, the old threshold is cut out. It is important to make cuts strictly along the intended lines, being careful not to touch the inside of the body. Often the old threshold consists of an outer and an inner part, and both need to be replaced if the inner one is also damaged by corrosion.
When cutting out old metal, leave small allowances that can be cleaned up later. If the rot has gone deep into the counter or floor, you will have to cut out these areas, welding them with new repair pieces. This is a labor-intensive process that requires restoring the geometry of the body. After completely removing the rusty parts, carefully clean the edges of the remaining metal until it shines, removing all traces of paint, primer and rust, as welding on dirty metal is unacceptable.
What to do if the stand is rotten?
If corrosion has affected the lower part of the rack, it must be cut out along with the threshold. To restore the geometry, use a slipway or powerful ties to prevent the body from “moving” during welding. The new post insert must be fitted with minimal gaps (2-3 mm) for good weld penetration.>
Pay special attention to the inside of the threshold. It often happens that there are no holes on the outside, but everything is rotten on the inside. The internal insert can be installed through technological holes or by cutting out part of the external threshold. To access hidden cavities, it is sometimes necessary to drill additional holes, which are then welded. The quality of preparation of the internal surface determines how long the car will live after repair without new rust.
Technology for installing and welding new thresholds
Installing a new insert begins with careful fitting. The part must adhere to the body around the entire perimeter without gaps or steps. Secure the new threshold with clamps and check the gaps with the door and fender. If the geometry is broken, use body hammers and straightening mandrels. Correct docking is 90% of success, since it will be almost impossible to digest an incorrectly installed part without deforming the body.
The welding process should be carried out in spots, and not in a continuous seam, to avoid overheating of the metal and its warping. Cook in short 2-3 cm sections, allowing the metal to cool. Tack the corners and center of the sides first, then fill in the gaps. For semi-automatic welding It is optimal to set the wire feed speed and voltage so that the penetration is complete, but without burning through. The seam should be smooth, without pores or sagging.
After welding, all seams must be cleaned with a flap wheel or file so that they are level with the surface of the body. This is necessary so that no unevenness is visible during painting. If you use a copper pad, it will help prevent burns in thin areas. It is also important to weld all accessible places from the inside, if possible, ensuring the tightness of the connection between the outer and inner inserts.
☑️ Welding quality control
Do not forget about corrosion protection during welding. It is impossible to prime the metal at the seam before welding, as the zinc will burn and the seam will be of poor quality. However, immediately after the seam has cooled and stripped, it is necessary to coat the exposed metal with epoxy primer. This will prevent instant oxidation of the stripped metal. Epoxy primer creates reliable insulation from moisture and oxygen, being the best base for painting.
Anti-corrosion treatment and painting
Finishing and protection is what distinguishes professional repairs from garage repairs. After welding and initial priming, the surface is puttied, if there are small irregularities, then primed with acrylic primer and painted. It is better to select paint using computer selection, taking into account the age of the car and possible fading of the original color. Before painting, be sure to cover all adjacent elements with masking tape and paper: doors, wings, glass and optics.
However, the most important part is the anti-corrosion treatment of hidden cavities. The threshold is a closed structure, and if you leave the inside unprotected, it will rot from the inside within a couple of years, even with an ideal appearance. For this purpose, special wax compounds or “cannon lard” are used, which are applied through a tube with a pressure sprayer. Processing must be thorough so that the composition covers all hard-to-reach corners.
| Material | Purpose | Application method | Service life |
|---|---|---|---|
| Epoxy primer | Insulating metal from moisture | Brush, spray gun | 10+ years |
| Anti-gravel | Protection against stones from below | Aerosol, pistol | 3-5 years |
| Movil / Voski | Protection of hidden cavities | Tube with spray | 2-3 years |
| Bitumen mastic | Noise insulation and underbody protection | Brush, spatula | 5-7 years |
It is also recommended to treat the outer part of the threshold from below with anti-gravel. This coating creates a thick, elastic layer that absorbs the impacts of fine gravel and sand flying from under the wheels. Apply it in several layers with intermediate drying. After drying, the anti-gravel can be painted over to match the body color or left black, which is standard practice for the lower part of the sills of many cars.
Geometry restoration and assembly
After all welding and painting work is completed, it is necessary to check the geometry of the door installation. When replacing the thresholds, especially if the welding was carried out without a slipway, the body could move a little. Doors may begin to close with force or, conversely, have large gaps. Adjusting the hinges can correct minor misalignments, but if the gaps are critical, additional adjustments may be required.
The car is reassembled in the reverse order. All removed interior elements that may have been damaged or dismantled for access are reinstalled. Pay special attention to door seals. It is better to replace old, stiff rubber bands with new ones, as they ensure the tightness of the interior and proper air circulation. A loose seal will allow water to enter the interior and rapid development of corrosion from below.
Check the operation of all electrical components in the doors: power windows, locks, speakers. When welding, wires often burn out or fuses burn out. If after welding something does not work, do not rush to disassemble everything again - first check the fuses and the integrity of the wiring in the harnesses going through the threshold. Often it is enough to restore contact in a connector that may have melted due to heat.
⚠️ Attention: Do not operate the car immediately after painting. Paints and varnishes must be completely polymerized, this can take from several days to two weeks depending on the temperature and type of paint.
High-quality anti-corrosion treatment is more important than perfect painting, since rotting begins from the inside, where it is not visible.
Common mistakes and how to avoid them
One of the most common mistakes is saving on metal. Using regular ferrous metal instead of galvanized steel or using too thin steel results in repairs having to be redone after a year or two. Also, craftsmen often neglect high-quality cleaning of edges, leaving oxides, which leads to rapid destruction of the weld. The seam must be clean and smooth, without slag.
Another mistake is the lack of ventilation when painting and drying. If you dry paint in a dusty garage without heating, dust will settle on the surface, and in the cold season, “shagreen” or dullness may appear. In addition, many people forget about the drainage holes at the bottom of the threshold. Under no circumstances should they be painted over or clogged with anti-corrosion agent, as condensation moisture must flow out through them.
The wrong choice of welding current also plays a role. Too much current will burn holes, and too little current will not give good penetration, creating only a superficial connection that will burst at the first load on the body. Practice on scrap metal before working on the car. This will help you select the optimal welding modes for your specific metal thickness.
- 🚫 Never weld “to tear off” - the metal should fit tightly along the entire contact plane.
- 🚫 Do not ignore priming the ends of the metal, even those that will go under hidden cavities.
- 🚫 Do not use putty to smooth out large differences - it will rub and fall off.
FAQ: Frequently asked questions
How long does it take to overcook thresholds in a garage?
The time depends on the qualifications of the technician and the degree of corrosion. One side usually takes 1-2 days if all the tools and materials are available. This includes dismantling, preparation, welding, filling and painting. If restoration of racks or floors is required, the time may increase to 3-4 days per side.
Is it possible to weld the thresholds without removing the doors?
Theoretically it is possible, but it is highly not recommended. Doors impede access and are easily damaged by sparks or tools. In addition, without removing the doors, it is impossible to accurately check the gaps and installation geometry of the new threshold. Removing doors is a mandatory step in professional repairs.
Do I need to remove the interior to replace the thresholds?
Complete dismantling of the interior is rarely required. Usually it is enough to remove the carpets, seats and plastic trim from the center console to gain access to the sill mounting points from the inside and carry out the wiring. In some cars, access is also possible through access hatches.
What metal is best to use for inserts?
The ideal option is galvanized steel of the same thickness as the factory one (usually 0.8-1.0 mm). Galvanization provides additional corrosion resistance. If this is not possible, use high-quality structural metal and pay maximum attention to anti-corrosion treatment.
What to do if the body moves after welding?
For minor misalignments, adjusting door hinges and locks will help. If the deformation is severe, straightening will be required on a slipway or using hydraulic stretchers. In difficult cases, it may be necessary to heat the metal and its deposit with a hammer, but this requires experience in order not to release the metal.