How to repair holes in thresholds: professional repair methods

Sill corrosion is perhaps the most common and painful problem for car owners, especially in harsh climates and the use of reagents on the roads. If you notice blistering paint or already through holes in the lower part of the body, you cannot hesitate for a day, as the rot is spreading at an alarming rate. Seal holes in thresholds There are several ways, the choice of which depends on the degree of damage, your budget and the availability of welding equipment.

There is an opinion that the threshold is a purely decorative element, but in fact it is an important part of the load-bearing frame of the body, ensuring torsional rigidity of the structure. Ignoring the problem will not only lead to loss of presentation, but also to a serious weakening of the vehicle’s safety in the event of a side impact. In this article we will analyze in detail how to qualitatively restore the integrity of the metal, using both classical welding and modern composite materials.

Before taking active action, it is necessary to soberly assess the scale of the disaster. Often behind a seemingly small hole there is hidden a huge “surprise” of loose rust, occupying a large area of ​​the internal cavity. High-quality repair always begins with a thorough diagnosis and cleaning to understand where the living metal ends and the rot begins.

Diagnosis of damage and assessment of scope of work

The first step is always a visual and tactile inspection. Tapping with a screwdriver or a special hammer helps to identify hidden pockets of corrosion that have not yet come out, but have already weakened the structure of the metal. If the tool easily pierces the surface or the metal is dented when pressed with a finger, it means that the process has gone too far and requires radical intervention.

It is important to determine whether the interior of the threshold is affected. To do this, you often have to drill technological holes or use an endoscope. If only the outer “shelf” is rotten, repairs will be cheaper and easier. If the power element is destroyed, then filling the holes turns into a complex operation of overcooking the entire assembly.

⚠️ Attention: Never seal through holes simply with putty or foam without pre-treatment. This will create an ideal greenhouse for moisture, and after a couple of months the rust will come out again, but in a much larger volume.

The assessment also includes checking adjacent elements: arches, underbody and pillars. Corrosion often spreads from the thresholds to neighboring parts, and if you miss this moment, all the work will go down the drain. Integrated approach diagnostics allows you to draw up an accurate work plan and purchase the necessary materials.

Preparation of tools and selection of materials

The success of the operation directly depends on what you do with your work. For the classical method you will need semi-automatic welding (MIG/MAG), an angle grinder (“grinder”) with a set of discs, clamps and, of course, new metal for insertion. The use of gas welding is not recommended, as it causes severe overheating, which can damage the geometry of the body.

If welding is not available or skills are lacking, chemical methods come to the rescue. You will need epoxy resins, fiberglass cloth (fiberglass), degreaser, rust converter and a quality putty. For finishing you will need primers, paints and varnishes that match the color of your car.

• 🛠️ Tools: Grinder, drill, hammer, chisel, welding machine, compressor.
• 🧪 Chemistry: Degreaser, rust converter, epoxy primer, fiberglass putty.
• 🛡️ Protection: Respirator, glasses, gloves, protective clothing (working with chemicals and metal dust is dangerous).
• 🧶 Materials: Sheet metal (0.8-1 mm), fiberglass, anti-corrosion compounds.

Pay special attention to the choice of metal for the insert. It should be the same thickness as the original sills, or a little thinner for ease of welding. Using metal that is too thick will create unnecessary stress in the weld area, and using metal that is too thin will quickly burn out.

Mechanical method: cutting and welding of a new part

This is the most reliable and durable method that returns the body to factory strength. The essence of the method is to completely remove the rotted area and install a new, pre-prepared part in its place. Welding thresholds requires care so as not to overheat the metal and not break the geometry.

First, the cutting line is marked. It is important to capture healthy metal with a margin of 2-3 cm from the visible rust border. A rectangle or a complex shape is cut out to follow the contours of the damage. Then a patch is cut out from the new sheet, which should fit perfectly into place with minimal gaps.

Connection type	Benefits	Disadvantages	Recommended Application
Butt-butt	Aesthetically pleasing, minimal layer of putty	Difficult to do without burns, requires experience	Visible outer parts of the thresholds
overlap	Easier to cook, less risk of burning	Thick seam, may require more sanding	Internal hidden cavities, bottom
Lined	High durability, convenient for beginners	Double metal, risk of corrosion between layers	Repair of spars and power elements

When welding, it is important to use an intermittent weld ("tack weld"), allowing the metal to cool. This will prevent body deformation from thermal expansion. After welding, the seams are cleaned with a grinder and treated with anticorrosive. Weld joint quality critically affects the service life of the repair.

Repair without welding: using fiberglass and epoxy

For those who do not have a welding machine, or if the holes are located in hard-to-reach places, the method using fiberglass and epoxy resins. This method is often called “cold welding,” although the physical and chemical processes here are completely different. It allows you to create a durable, leak-proof and rust-resistant membrane.

The technology consists of layer-by-layer gluing of pieces of fiberglass impregnated with an epoxy composition onto a previously cleaned surface. It is important that the edges of the hole are cleaned down to live metal and have a “chamfer” shape for better adhesion. Fiberglass works as reinforcement, and resin acts as a binder and protective substance.

The process is as follows: a layer of resin is applied to a degreased surface, fiberglass is laid on top, then resin again, and so on several times (usually 3-5 layers). Each layer should slightly overlap the previous one. After drying, you get a “shell” that can be puttyed and painted.

⚠️ Attention: Epoxy resin is afraid of ultraviolet radiation and may turn yellow or crack over time if not protected with paint. Be sure to cover the repair area with varnish or enamel.

The main advantage of this method is the absence of thermal effects on the body, which eliminates metal leashes. However, such a patch does not have the same mechanical tensile strength as solid metal, so it may burst under strong impacts. For restoring tightness and stopping corrosion is an excellent solution.

The nuances of working with epoxy

The pot life of the mixture is limited to 20-40 minutes, so do not mix a lot of composition at once. You need to work quickly, but carefully. The room temperature should not be lower than +20°C, otherwise the resin may not polymerize or may take several days to dry.

Finishing and anti-corrosion protection

Once the hole is repaired, whether by welding or fiberglass, the finishing step begins. The surface is leveled with putty (it is better to use putty with aluminum powder or fiberglass for the first layer). This is followed by sanding, priming and painting.

But the most important thing is anti-corrosion treatment the internal cavity of the threshold. If you leave the metal inside without protection, it will rot from the inside in 1-2 years, even if the outside paint is perfect. For this purpose, special wax or oil compositions are used, which are introduced through technological holes using a sprayer with a long tube.

• 🚿 Movil: A traditional oil-based product that penetrates well, but dries out and cracks over time.
• 💧 Cannon fat: Long lasting, but very thick, difficult to apply evenly when cold.
• 🛢️ Modern waxes: Remain elastic, have a self-tightening effect, and do not flow when heated.

Treatment should be carried out regularly, once every 2-3 years, especially if the car is operated in winter conditions. High quality anticorrosive displaces moisture and creates a film that prevents metal from contacting oxygen.

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The quality of internal anti-corrosion treatment is more important than external painting. Rust always comes from the inside, where it is not visible until holes appear.

Typical mistakes when repairing thresholds

Many car enthusiasts, trying to save money, make mistakes that ruin all their efforts. The most common of these is poor cleaning of the hole edges. If you leave even a little loose rust, the rotting process will continue under a layer of putty or paint, and after a season the repair will have to be done again.

Another mistake is using regular construction foam to fill large ones. Foam is hygroscopic, that is, it absorbs moisture from the air, turning the threshold into a sponge that rots from the inside at twice the speed. To fill volumes, use only special closed-cell materials or dense packaging with an anticorrosive mixture.

Ventilation is also often overlooked. By welding all technological holes tightly, you seal the moisture inside. Air must circulate in the threshold, or it must be perfectly preserved with wax. Violation of technology leads to rapid failure of even well-made repairs.

Is it possible to repair a hole in the threshold using cold welding (metal)?

Two-component metal rods ("cold welding") are suitable only for temporary solutions or for sealing small fistulas in non-critical areas. They do not have the necessary elasticity and adhesion for large threshold areas. Under the influence of body vibrations, such a patch will quickly fall off.

How much does it cost to seal thresholds at a service?

The price varies greatly depending on the region and method. Partial overcooking of one side can cost from 10,000 to 25,000 rubles. A complete replacement of thresholds with painting and anti-corrosion on a modern car often exceeds 50,000 - 80,000 rubles, which makes independent repairs economically feasible.

Do I need to remove the interior to replace the thresholds?

In 90% of cases - yes. To get good access to the places where the threshold is welded to the pillars and the bottom, it is necessary to dismantle the seats, carpets and sometimes even the center console. An attempt to replace the threshold “without removing the interior” almost always leads to poor welding quality and hidden pockets of corrosion.

What to do if the floor under the thresholds has rotted?

If the rot has reached the floor, repairs become more difficult. You will have to cut out part of the floor, install reinforcements and a new floor panel. This is already a serious body repair that requires a slipway or very precise geometry so as not to disturb the mounting points of the suspension and engine.