Welding car sills: a complete guide from preparation to finishing

Thresholds are one of the most vulnerable parts of a car body. They are constantly attacked by moisture, salt, sand and mechanical damage, which leads to corrosion and through holes. If you notice rust or deformation on the sills of your car, do not put off repairs: timely welding will help avoid costly restoration of the entire body. In this article we will look in detail at how to weld thresholds on a car with your own hands - from the choice of equipment to the final anti-corrosion treatment.

Welding thresholds requires not only skills in working with a welding machine, but also an understanding of the characteristics of the metal, the ability to prepare the surface and properly protect it after repair. We will look at all stages of the process: how to determine the extent of damage, what materials and tools will be needed, how to properly cut out rust and weld patches or new thresholds. You will also learn about common mistakes that beginners make and how to avoid them.

Important: if you have never worked with a welding machine before, practice on unnecessary pieces of metal before starting to repair your car. The quality of the weld directly affects the strength of the body and the safety of the vehicle as a whole.

When threshold welding is required: signs and diagnosis

The first step is to correctly assess the condition of the thresholds. Rust on the surface does not always mean that the metal is rotten through and through. Here are the key signs that the thresholds require welding:

• 🔍 Through holes - If you see holes or can pierce the metal with a screwdriver, welding is inevitable.
• 🚗 Blistering paint — bubbles under the paintwork indicate corrosion from the inside, even if the metal looks intact from the outside.
• 💧 Dampness inside the threshold — if after rain there is a smell of mold in the cabin or water under your feet, the sills are probably rotten.
• 🔧 Deformation when pressed - if the metal bends under your hand, its strength is critically reduced.

For an accurate diagnosis, use endoscope (flexible camera) or simply shine a flashlight inside the threshold through the technological holes. Please note threshold amplifiers - internal metal beams, which often rust first. If they are destroyed, a complete replacement of the threshold will be required, rather than local repairs.

Tip: before starting work, take a photo of the thresholds from all sides and make marks on the metal with a marker - this will help when fitting new parts. Also check the condition jacking zones — they often suffer along with thresholds.

Necessary tools and materials: complete checklist

50% of success depends on the quality of the tools. Do not skimp on key positions - a cheap welding machine or unsuitable electrodes can ruin even perfectly prepared metal. Here's what you'll need:

Angle grinder (grinder) with cutting and grinding wheels

Welding machine (semiautomatic MIG/MAG or inverter for MMA)

Chameleon protective mask or light filters

Fireproof gloves and clothing

Hammer, chisel, screwdriver set

Jack and stands for fixing the body

New thresholds or sheet metal for patches (thickness 0.8–1.2 mm)

Primer, putty, anti-corrosion compound (for example, ML-mastic)

Paint in cans or a compressor with a spray gun-->

When choosing a welding machine, give preference semi-automatic MIG/MAG - it is easier to learn and produces a more accurate seam than manual arc welding. To work with thin body metal, use a wire with a diameter of 0.6–0.8 mm and gas Ar/CO₂ (80/20). If your budget is limited, you can get by with an inverter for MMA welding, but then you will need electrodes ∅2 mm and experience working with thin metal.

To protect against corrosion after welding, prepare:

• 🧴 Acidic soil (for example, Reoflex) - for processing welds.
• 🎨 Epoxy primer - as an insulating layer before painting.
• 🛡️ Anti-gravel coating - to protect the lower part of the thresholds.
• 🔥 Heat-resistant mastic - for processing internal cavities.

⚠️ Attention: Never use for welding thresholds carbon dioxide (CO₂) in its pure form - it gives a too “hard” arc, which leads to burns through thin metal. The optimal mixture is argon with 18–20% CO₂.

Vehicle preparation: disassembly and protection

Before welding, the car must be prepared so as not to damage other elements and to ensure access to the thresholds. Start with:

1. Withdrawals doors (if you need to completely replace the threshold) or at least unscrew them and fix them in the open position.
2. Dismantling interior trim, thresholds and seals. Be careful with the caps - they often break.
3. Shutdowns battery and withdrawals fuse box, if it is located near the welding zone.
4. Closings glass, headlights and other fragile elements fire-resistant material (for example, asbestos sheet).

Pay special attention grounding. The ground of the welding machine must be connected to bare metal body, and not to the threshold or other removable elements. Poor contact will result in "stray currents" that can damage the vehicle's electronics.

If you are working in a garage, ensure good ventilation - welding produces toxic gases (ozone, nitrogen oxides). Also prepare in advance fire extinguisher — sparks from an angle grinder or welding can ignite oil or other flammable materials.

Threshold welding technology: step-by-step instructions

Now we move on to the most important stage - welding. Let's consider two options: local repair (patches) and complete threshold replacement.

1. Local repairs (patches)

Suitable if rust has not affected the amplifiers and the main structure of the threshold. Procedure:

1. Bulgarian cut out rusty area, retreating to 10–15 mm from the visible corrosion limit. Clean the edges of the cut until shiny.
2. Prepare patch made of sheet metal (thickness must match the original). Place it on the hole and circle it with a marker.
3. Grab the patch spot welding in 3-4 places, then weld the seam in a continuous line. Use chess orderto avoid overheating of the metal.
4. Clean the seam sander and process acid soil.

2. Complete replacement of the threshold

If the threshold is more than 60% rotten or the amplifier is destroyed, a complete replacement will be required. What's important here is:

• 🔧 First remove the old threshold, carefully cutting it off with a grinder. Don't damage it spars and body pillars.
• 📏 Customize new threshold to size, trimming it if necessary. Check fit along the entire length.
• 🔥 Start welding with potholders (short seams 1–2 cm long) every 10–15 cm. Then weld the seams completely, alternating sides to avoid deformation.
• 🛠️ After welding threshold amplifier (if it is removable) is welded separately, with mandatory processing of internal cavities anticorrosive.

It is critically important: when completely replacing a threshold, never weld it “by weight” - fix the body on a slipway or use spacers to avoid distortion of the geometry. If there is no slipway, you can temporarily weld it to the thresholds corners for rigidity, which are then cut off.

Welding type	Benefits	Disadvantages	Recommendations
MIG/MAG (semi-automatic)	Neat seam, easy to control, suitable for thin metal	Requires a gas cylinder, more expensive than an inverter	Optimal for beginners and professionals
MMA (manual arc)	Cheaper, does not require gas, suitable for thick metal	It is difficult to weld thin metal, there is a lot of spatter	For experienced welders only
TIG (argon arc)	The highest quality seam, minimal deformation	Difficult to learn, expensive equipment	For professional body repair
Spot welding	Minimal heating, no deformation	Not suitable for continuous seams, requires special equipment	For tack work and local repairs

⚠️ Attention: When welding thresholds on cars with galvanized body (for example, Volkswagen, Audi, Skoda) use wire with high silicon content (SG2 or SG3). This will prevent cracks from forming in the seam due to differences in thermal expansion coefficients.

Anti-corrosion treatment after welding: how to protect thresholds for a long time

Even a perfect weld will quickly rust without proper treatment. Anti-corrosion protection consists of several stages:

1. Cleaning - remove slag and scale from the seam with a wire brush or grinder. Degrease the surface white spirit.
2. Primer - apply acid soil (for example, Reoflex Acid Primer) on welds and exposed metal. It converts residual rust and ensures adhesion.
3. Isolation - cover primed areas epoxy primer (for example, Novol Protect 360). This will create a moisture barrier.
4. Protection of internal cavities - fill through the technological holes ML-mastic or wax anticorrosive (for example, Tectyl). Use anticorrosive gun with flexible nozzle.
5. Final painting - Apply 2-3 layers of paint with intermediate drying. For thresholds it is recommended to use acrylic paint with anti-gravel properties.

Pay special attention lower part of the threshold — it suffers most from sandblasting and chemical reagents. Here you can additionally apply anti-gravel film or liquid rubber (for example, Plasti Dip). Also check the integrity drainage holes - if they are clogged, moisture will accumulate inside the threshold.

What happens if you skip anti-corrosion treatment?

Without protection, welds will begin to rust within 3–6 months, especially in winter. Corrosion will spread under the paint, and after 1-2 years the thresholds will have to be welded again. In addition, rust can spread to the side members and struts, which will lead to loss of body rigidity and problems with passing inspection.

Typical mistakes when welding thresholds and how to avoid them

Even experienced craftsmen sometimes make mistakes that then have to be corrected. Here are the most common:

• 🔥 Metal overheating — leads to deformation of the threshold and weakening of the seam. Solution: cook in short seams (3–5 cm) with breaks for cooling.
• 🧲 Poor cleaning before welding — residues of rust or paint lead to porosity of the seam. Solution: use sandblaster or a sander with an attachment Scotch-Brite.
• ⚡ Incorrect grounding — leads to “stray currents” and damage to electronics. Solution: connect the ground to the bare metal of the body, and not to the threshold.
• 📏 Incorrect fit of patches - more gaps 1 mm lead to a weak seam. Solution: use clamps for fixing parts before welding.
• 🎨 Skipping primer — paint directly on metal will quickly peel off. Solution: always apply two-component primer before painting.

Another common mistake is ignoring body geometry. After replacing the thresholds, check:

• 🚪 Gaps between doors and thresholds (must be the same along the entire length).
• 🔄 Symmetrical wheel arches (measure the diagonals between the control points).
• 📐 Rack angles (use laser level or templates).

If the geometry is broken, the car will handle worse and the doors may stop closing. To check use measuring system (for example, Car-O-Liner) or at least a tape measure and a cord.

Cost of work: should I cook it myself or send it to a service?

The price of welding thresholds depends on the degree of damage, car brand and region. Here are the estimated prices for services (for 2026):

Type of work	Cost (RUB)	Lead time
Local repair (1–2 patches)	3 000 – 8 000	2–4 hours
Partial threshold replacement (up to 50%)	8 000 – 15 000	4–6 hours
Complete threshold replacement (with amplifier)	15 000 – 30 000	6–10 hours
Anti-corrosion treatment (comprehensive)	2 000 – 5 000	1–2 hours

If you decide to weld the thresholds yourself, your costs will be limited to the cost of materials:

• 🔧 New threshold (for example, for VAZ 2110) — 1,500–3,000 rub.
• 🧴 Primer, paint, anticorrosive - 2,000–4,000 rub.
• ⚡ Electrodes/wire — 500–1,500 rub.
• 🛠️ Consumables (grinder wheels, sandpaper) — 300–800 rub.

Total: DIY repairs will cost 4,000–9,000 rub. against 15,000–30,000 rub. in the service. However, keep in mind that without experience you may make mistakes that will then have to be corrected by professionals. If you have never welded thin metal, it is better to practice on unnecessary parts or contact service.

FAQ: Frequently asked questions about welding thresholds

Is it possible to weld thresholds on a car with electronics (for example, Toyota Camry or BMW)?

Yes, but you need to take precautions:

1. Disconnect the battery and remove body control module (BCM), if it is located nearby.
2. Use spot welding or semi-automatic with minimal currentto avoid power surges.
3. Connect the ground of the welding machine as close as possible to the welding site.
4. After work, check for errors with a scanner (for example, ELM327).

On vehicles with aluminum body (for example, Jaguar, Audi A8) welding of thresholds requires argon arc welding (TIG) and special skills - here it is better to contact the service.

How to weld thresholds on a galvanized car without damaging the coating?

Galvanized metal requires a special approach:

• Use wire with high silicon content (SG2 or SG3).
• Reduce wire feed speed by 10–15% compared to conventional steel.
• After welding, be sure to treat the seam zinc-containing primer (for example, Zinc Rich Primer).
• Do not overheat the metal - galvanization loses its properties at temperatures above 400°C.

By car Volkswagen, Skoda, Ford (with factory galvanization) it is better to use gas welding or spot weldingto minimize the heating zone.

What to do if the doors do not close well after welding the thresholds?

This means that the geometry of the body is broken. You can fix the problem like this:

1. Check gaps between the door and the threshold - they must be the same along the entire length (3–5 mm).
2. Measure diagonals between control points (for example, from the corners of the hood to the trunk). The difference should not exceed 5 mm.
3. If the threshold is installed crookedly, cut off the welds and reinstall it using clamps and spacers.
4. As a last resort, contact slipway to restore geometry.

Don't try to "fit" the doors with a saw or hammer - this will only make the problem worse. The reason is almost always that the threshold is set incorrectly.

How long will thresholds last after welding?

Service life depends on three factors:

1. Welding quality - if the seams are welded through and there are no voids, the thresholds will last 5–10 years.
2. Anti-corrosion treatment — with proper protection (primer + mastic + painting), the metal will not rust 7–12 years.
3. Operating conditions — in regions with abundant use of reagents (Moscow, St. Petersburg), thresholds wear out faster.

To extend the life of thresholds:

• Update once a year anti-corrosion coating.
• Wash your car at least once every 2 weeks in winter.
• Install mud flaps and wheel arch liners.

Is it possible to weld sills on a car with airbags?

Yes, but with caution. Airbags (airbag) will not work from welding, however:

• Disconnect the battery and wait for the capacitors to discharge (10–15 minutes).
• Don't cook near squibs pillows (usually they are located in the pillars and steering wheel).
• Use minimum currentto avoid voltage surges in the on-board network.
• After work, check for errors with a scanner - sometimes welding can damage shock sensors.

On vehicles with side cushions (in doors or seats) it is better to remove the trim and disconnect the squib connectors before welding.