Body corrosion is not just an aesthetic problem, but a serious threat to the safety and marketability of your car. Exactly rapids take the brunt of moisture, reagents and mechanical damage, being one of the first elements to rot. If you notice blistering paint or through holes at the bottom of the doors, you can no longer put off repairs.
In this article we will analyze in detail the process of replacing thresholds, which can be done in a garage with a minimum set of tools. We'll discuss material selection, welding techniques, and most importantly, methods to protect new metal from rusting again. A competent approach will allow you to save a significant amount compared to going to a body shop.
It is worth remembering that replacing thresholds requires not only technical skills, but also an understanding of the design monocoque body. Improper work can disrupt the geometry of the vehicle and reduce its strength in a side impact. Therefore, carefully study the theoretical part before taking up the grinder.
Diagnosis of condition and selection of material
The first step is always a thorough diagnosis. Often the external condition of the paintwork hides the extent of the problem. It is necessary to tap the lower part of the thresholds and compare the sound with healthy areas; A hollow sound may indicate peeling metal or hidden corrosion. It is also worth paying attention to the condition of the side members, since rotting of the sills often spreads to the power elements.
For replacement, you can use several types of materials, each of which has its own characteristics. The main selection criterion is the quality of the steel and the presence of factory galvanizing. Using ordinary ferrous metal without protection will result in the procedure having to be repeated after a year or two.
- 🔩 Original thresholds: Ideally follow the geometry, but are expensive and often require a long wait for delivery.
- 🔩 Analogs made of galvanized steel: Optimal choice in terms of price and quality ratio, provide good corrosion protection.
- 🔩 Stainless steel linings: Used for decorative protection, but do not replace the power part of the threshold in case of through corrosion.
⚠️ Attention: When purchasing non-original thresholds, be sure to check the thickness of the metal with a caliper. Steel that is too thin (less than 0.8 mm) will deform during welding and quickly burn out.
When choosing a material, it is important to consider whether the threshold will be a load-bearing element. On most modern cars, the sills are part of the load-bearing structure of the body, so it is absolutely impossible to save on metal here. The use of used thresholds from disassembly is permissible only if they are completely free of rust and have a residual metal thickness close to the factory one.
Necessary tools and workplace preparation
High-quality replacement of thresholds is impossible without suitable equipment. You will need not only a standard auto mechanic kit, but also specialized tools for body work. Particular attention should be paid to personal protective equipment, since the work involves sparks, metal dust and harmful fumes.
The main tool for dismantling and cutting will be an angle grinder, popularly called a “grinder”. For welding work, a semi-automatic machine (MIG/MAG) is best suited, as it allows even beginners to make high-quality seams, unlike manual arc welding. You will also need clamps to secure the parts and clamps.
☑️ Preparation for welding work
Don't forget to prepare the surface in advance. This includes washing the car, especially the arches and underbody, and removing sound insulation if it interferes with access to the mounting points. It is best to work on level ground or in a garage with good lighting and ventilation.
Technology for dismantling old thresholds
The process of removing rotted elements requires care so as not to damage the surviving parts of the body and attachments, such as doors and fenders. Before cutting begins, it is necessary to dismantle the doors or ensure that they are securely fixed in the open position so that they do not interfere with access to the bottom of the opening.
Cutting the old threshold should be done along the line of the factory seam or slightly retreating from it if the metal is severely damaged. It is important not to touch the internal reinforcements and side members, if they are preserved. In some cases, it is necessary to cut the threshold in parts so as not to disrupt the rigidity of the structure during the process.
- 🔨 Marking cutting lines: Clearly mark the boundaries of the area to be removed with chalk or a marker.
- 🔨 Protection of adjacent areas: Cover the adjacent body panels with masking tape and a rag.
- 🔨 Accurate cutting: Use a thin cutting wheel to minimize metal heating and distortion.
After removing the external part of the threshold, it is often discovered that the internal amplifier also requires replacement or repair. Thoroughly scrape all surfaces down to bare metal using a coral drill bit or flap wheel. Only on a perfectly clean surface can new metal be applied.
Installation and welding of new elements
Installation of new thresholds begins with fitting. The part should fit tightly around the entire perimeter without gaps. If the geometry of the purchased threshold does not match the body, it must be adjusted using a mallet and clamps. The gaps between the threshold and the door must be uniform along the entire length.
Welding should be done in spots or short seams, allowing the metal to cool to avoid overheating and "behavior" of the body. Start cooking from the center to the edges or from one of the edges, moving progressively. It is important to weld all joints, especially at the junction with the side members and struts.
Secrets of quality welding
When welding thin-walled metal, use reverse polarity and reduce wire feed. This will avoid burns. It is also recommended to use a copper backing under the seam for better heat dissipation.
After completing the welding work, it is necessary to clean the seams. They must be smooth and free of bulges that could interfere with the installation of plastic covers or create areas for future corrosion. Use a sanding wheel to smooth the surface.
Before final welding of the threshold, tack it in several places and hang the door. Check the clearances and operation of the lock. If everything is in order, you can finally cook.
Anti-corrosion treatment and protection
The most critical stage on which the durability of the repair depends is anti-corrosion treatment. New metal, devoid of the factory zinc layer (if it is not galvanized), is susceptible to corrosion even more than the old one. Therefore, protection must be comprehensive and multi-layered.
First, the surface is degreased and coated with an acid primer (phosphating), which provides adhesion and chemical protection. Then an epoxy primer is applied, creating a sealed film that does not allow moisture to pass through. Only after this can the final paint coating be applied.
Pay special attention to the internal cavities of the thresholds. Through special technological holes or drilled holes it is necessary to pump anti-corrosion composition (e.g. wax or oil based) using a gun with a long nozzle. This will create a protective layer from the inside.
| Coverage type | Function | Service life | Application Features |
|---|---|---|---|
| Acidic soil | Chemical protection, adhesion | 5-7 years | Thin layer, requires overlapping |
| Epoxy primer | Sealing, insulation | 10+ years | Not hygroscopic, not afraid of moisture |
| Anti-gravel | Stone impact protection | 3-5 years | Apply to external parts |
| Movil/Cannon fat | Protection of internal cavities | 2-4 years | Requires periodic updates |
Finishing and painting
After priming comes the painting stage. The color is selected according to the body code, but due to fading of the old coating, repainting may be required by transferring to adjacent elements (doors, fenders) to perfectly match the tone. If the thresholds are black or covered with overlays, it is enough to cover them with black enamel or anti-gravel.
When painting, it is important to observe the temperature regime and drying time between layers. Apply varnish in 2-3 layers to obtain depth of color and gloss. After the varnish has dried, the surface is polished to remove dust particles and shagreen.
If you do not plan to paint the sills in the body color, but will install plastic trims, make sure that the metal underneath is reliably protected. Plastic creates a greenhouse effect, and any scratch on unprotected metal will quickly turn into a source of rust.
The quality of painting and anti-corrosion treatment is more important than the ideal geometry of the welds, since it is the paint layer that protects the metal from the environment.
Common mistakes and expert advice
Beginners often make mistakes that ruin all their efforts. One of the most common is poor cleaning of edges before welding, which leads to lack of penetration and rapid rotting of the joints. They also often forget about internal treatment, protecting only the outer side of the threshold.
Another mistake is using the wrong materials to fill voids. Some try to blow out polyurethane foam or pour in bitumen, which is strictly prohibited. Foam absorbs moisture like a sponge, and bitumen cracks in the cold, letting water through to the metal.
- ❌ Ignoring ventilation: Blockage of all openings leads to condensation inside the threshold.
- ❌ Metal overheating: Long-term welding without interruptions deforms thin body metal.
- ❌ Saving on materials: Cheap primers and paints may not have the necessary anti-corrosion properties.
⚠️ Attention: Never completely close the service openings at the bottom of the thresholds. They are necessary for condensate drainage and ventilation of the internal cavity. Use special drainage valves or meshes.
Following technology and using quality materials will allow you to forget about the problem of rusty thresholds for many years. Don’t be afraid to ask questions on forums or consult with experienced professionals if you doubt your actions.
Do the doors need to be removed to replace the thresholds?
In most cases, it is not necessary to remove the doors, but this greatly facilitates access to the welding areas and avoids damage to the hinges or paintwork from sparks. If you work alone, removing the doors is a smart decision.
Is it possible to replace the thresholds without welding, with bolts?
Theoretically, it is possible, but this violates the rigidity of the body and reduces safety in a side impact. A bolted connection is also more difficult to seal against moisture, which will accelerate corrosion. Welding is the only correct method.
How long does it take to replace one threshold?
For an experienced craftsman with all the tools and materials, replacing one threshold takes 4-6 hours of pure time. For a beginner, taking into account adjustments and training, the process can take 1-2 days.
What to do if the pillars and thresholds have rotted?
If corrosion has affected the racks (especially the lower part), the repair becomes more difficult and requires partial rewelding of the racks. This is a highly complex job that requires maintaining the geometry of the body, and in such cases it is better to contact a professional body center.