How to properly repair holes in car sills: from temporary repairs to major repairs

Holes in car sills are one of the most common problems faced by owners of used cars. This is especially true for cars older than 5-7 years, where corrosion has had time to do its work. But even on relatively new cars, through holes may appear due to mechanical damage, unsuccessful repairs or manufacturing defects.

Many car owners put off repairing thresholds “for later”, not realizing that even a small hole can become a source of rust, which will quickly spread to the load-bearing elements of the body. In this article we will look at all possible ways to fill holes - from temporary measures to professional repairs with a guarantee of durability. You will learn which materials are suitable for different types of damage, how to properly prepare the surface and avoid mistakes that lead to the re-occurrence of corrosion.

Important: the repair method depends not only on the size of the hole, but also on the material of the threshold (steel, aluminum), its design (welds, reinforcements) and even climatic operating conditions. For example, cars that frequently drive on salty roads in winter will require more radical protection measures than cars from a dry climate.

Why do holes appear in thresholds and why are they dangerous?

The main reason for the formation of through holes is metal corrosion, which develops due to several factors:

• 🔹 Humidity and salt: winter reagents corrode the protective coating, exposing the metal. Even microcracks in paint become hotbeds of rust.
• 🔹 Mechanical damage: stone impacts, unsuccessful parking or an accident damage the integrity of the threshold.
• 🔹 Poor factory processing: on budget models (for example, Renault Logan or Lada Granta) anti-corrosion protection is often insufficient.
• 🔹 Poor quality previous repairs: if the hole is simply covered with putty without treating the metal, corrosion will continue to develop under the layer.

The danger of holes in thresholds is often underestimated. In fact they lead to:

• 🚗 Loss of body rigidity — the thresholds are a power structure, and their destruction impairs controllability.
• 🔧 Spread of rust to side members — if the problem is not fixed in time, repairs will cost many times more.
• 💰 Reducing the cost of a car when selling - even small holes become an argument for bargaining.

⚠️ Attention: If the hole in the sill is located near the mounting location of the suspension or power unit, operate the vehicle absolutely not possible - this can lead to deformation of the body when hitting an obstacle.

How to assess the extent of damage: when you can do without welding

Before choosing a repair method, you need to honestly assess the condition of the threshold. Here are the criteria to help you decide:

Hole size	Depth of corrosion	Recommended Method	Cost (on your own)
Up to 1 cm	Surface rust	Putty + primer	300–800 ₽
1–3 cm	Through corrosion, but the metal around is intact	Rivets + epoxy resin or fiberglass	800–1 500 ₽
3–10 cm	Rust has taken over the edges, the metal is crumbling	Metal patch (welding or rivets)	1 500–3 000 ₽
More than 10 cm	The threshold is rotten through, rigidity is lost	Replacing a threshold or part of it	From 5,000 ₽ (with work)

For an accurate diagnosis you will need:

• 🔦 Flashlight - to inspect the inside of the threshold through the technological holes.
• 🔨 Screwdriver or awl - pierce the edges of the hole: if the metal crumbles easily, the corrosion is deeper than it seems.
• 📏 Ruler - measure not only the diameter of the hole, but also the thickness of the remaining metal.

⚠️ Attention: If, when you press the threshold with your hand, it bends or makes a squeaking sound, this is a sign that the metal has lost its rigidity. In this case, cosmetic repairs will not help: you will need to install amplifiers or completely replace the element.

Method 1: Putty - fast, but short-lived

The putty is only suitable for small holes (up to 1–1.5 cm) without active corrosion around. This is a temporary solution that will last 1-2 years if the surface is properly prepared.

Step by step instructions:

1. Strip the paint and rust around the hole down to bare metal (use P80 sandpaper or grinder with a petal circle).
2. Treat the surface rust converter (for example, Tsinkar or Kudo KV-70007).
3. Apply 1-2 coats acid soil (it improves adhesion).
4. Stick on the back of the hole masking tape - this will prevent the putty from leaking out.
5. Fill the hole putty with fiberglass (for example, Novol Fiber or 3M 05893). Apply in layers of 2–3 mm, allowing drying between each layer.
6. After drying, sand the surface sandpaper P240→P400 and paint it.

🔹 Pros: cheap, can be done in 1–2 hours, does not require special equipment.

🔸 Cons: cannot withstand vibrations, may peel off over time, and does not stop corrosion.

Fiberglass putty (1 can)|Rust converter|Acid primer|P80, P240, P400 sandpaper|Making tape|Respirator and gloves-->

⚠️ Attention: Never putty holes larger than 2 cm - vibration will quickly crumble the putty. Also, do not use regular all-purpose putty (such as Novol Universal) - it cannot withstand the loads on the thresholds.

Method 2: Epoxy resin + fiberglass - semi-professional approach

This method is suitable for holes of size 1–5 cm and provides a more durable result than putty. Fiberglass reinforces the repair layer, and epoxy resin creates an airtight coating that does not allow moisture to pass through.

Algorithm of actions:

1. Strip the threshold down to the metal, removing all rust. Use drill with brush attachment for hard to reach places.
2. Degrease the surface acetone or white spirit.
3. Cut the patch from fiberglass (the size should cover the hole by 2-3 cm on all sides).
4. Apply a layer to the threshold epoxy resin (for example, Epoxy 520 or Poxipol), attach fiberglass and soak it with resin.
5. Repeat the process 2-3 times for strength.
6. After curing (24 hours), sand the surface and paint.

🔹 Benefits:

• 🛡️ Withstands vibrations and mechanical loads.
• 💧 Completely seals the hole, preventing further corrosion.
• 🔧 Can be combined with rivets for added strength.

🔸 Disadvantages:

• ⏳ Long drying time (up to 24 hours).
• 🧤 Requires working with gloves - epoxy resin is toxic.
• 🎨 It is difficult to achieve a perfectly smooth surface for painting.

Method 3: Rivets + metal patch - reliable and inexpensive

If the hole in the threshold has the size 3–10 cm, and the metal around is still preserved, the best option is to install a patch on the rivets. This method does not require a welding machine and is suitable for DIY repairs.

What you will need:

• 🔧 Sheet metal 0.8–1 mm thick (galvanized or aluminum is suitable).
• 🛠️ Riveter and aluminum rivets (diameter 3.2–4 mm).
• 🔨 Metal scissors or Bulgarian for cutting out the patch.
• 🧴 Seam sealant (for example, Terostat 922).

Step by step instructions:

1. Cut a patch from sheet metal that will cover the hole by 1.5–2 cm on all sides. Bend the edges of the patch by 5 mm for rigidity.
2. Apply the patch to the threshold and drill holes around the perimeter (drill diameter is 0.1 mm larger than the rivet diameter).
3. Treat both surfaces anti-corrosion primer (for example, Body 930).
4. Reinstall the patch and rivet, starting from the center. The pitch between rivets is 1.5–2 cm.
5. Tape the seam sealant and after drying, fill the uneven areas.

🔹 Advantages of the method:

• 🔩 Strength is almost like welding, but without the risk of burning through the metal.
• 💰 Cheaper than welding work (a riveter costs from 500 ₽, metal - 200–300 ₽ per sheet).
• 🔧 You can do it yourself without experience.

⚠️ Attention: Do not use steel rivets on aluminum thresholds (and vice versa) - this will lead to galvanic corrosion. For mixed metals, choose rivets from stainless steel.

How to avoid mistakes when working with rivets

1. Do not drill holes too close to the edge of the patch - the metal may crack.

2. Do not overtighten the rivets: they should “sit” tightly, but without deforming the sheet.

3. After installation, check the patch for strength: if it “plays” when pressed, add 2-3 more rivets in the corners.

Method 4: Welding is the most reliable, but not always accessible method

Welding is the only method that restores initial strength of the threshold. It is suitable for holes of any size, but requires skill and equipment. If you have never welded metal, it is better to entrust it to professionals.

Types of welding for thresholds:

• 🔥 Semi-automatic (MIG/MAG) — optimal for thin metal (0.8–1.2 mm). Use wire ER70S-6 with a diameter of 0.6–0.8 mm.
• ⚡ Argon (TIG) - for aluminum thresholds (for example, on Audi A4 or BMW 3-series). Requires an inverter with AC function.
• 🔧 Spot - Suitable for welding patches, but not for continuous seams.

Repair technology:

1. Cut out rusty metal Bulgarian or plasma cutter, leaving a margin of 5–10 mm for healthy metal.
2. Prepare pattern made of new metal (you can use a part from an old threshold or a purchased patch).
3. Tack the patch at 3-4 points, then weld continuous seam in increments of 1–1.5 cm.
4. Clean the seam sander, process primer for metal and paint it.

💡 Advice: If you are welding a threshold for the first time, practice on an unnecessary piece of metal of the same thickness. Optimal settings for a semi-automatic machine:

• Wire feed speed: 4–5 m/min
• Voltage: 18–20 V
• Gas consumption (CO₂/Ar): 8–10 l/min

⚠️ Attention: When welding thresholds You can’t weld straight through with long seams - this leads to metal deformation. Use the "checkerboard" method: short seams of 2-3 cm with cooling between them.

Method 5: Replacing the threshold - a radical solution

If the threshold is rotten by more than 30% or holes occupy most of its area, the only way out is full or partial replacement. This is the most expensive, but also the most reliable method that will return the car to its factory strength.

When replacement is required:

• 🚗 The threshold has lost its rigidity (flexes when pressed).
• 🔧 Rust has spread to spars or struts.
• 💥 The threshold was damaged as a result of an accident (folds, cracks).

How the replacement occurs:

1. Removed doors, bumper, trim (with complete replacement).
2. The old threshold is cut out Bulgarian or pneumatic tools.
3. The new threshold is adjusted to size, tacked and welded.
4. The seams are sanded and applied anti-corrosion treatment (for example, ML soil + mastic).
5. Amplifiers are installed (if provided for in the design).

💰 Cost:

• Threshold (new): from 3,000 to 15,000 ₽ (depending on the car model).
• Work: from 5,000 to 20,000 ₽ (complete replacement with painting).
• Additionally: anti-corrosion treatment - 1,500–3,000 ₽.

🔹 Where to buy thresholds:

• 🛒 Original — from official dealers (expensive, but perfect fit).
• 🔧 Analogues - in stores like Exist, Autodoc (less expensive, but may require some work).
• ⚙️ Used - at a showdown (risk of buying a rusty threshold).

⚠️ Attention: When replacing thresholds on vehicles with passive safety system (for example, Volvo or Toyota) use only original parts. An incorrectly set threshold can disrupt operation deformation zones in case of an accident.

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

Even the highest quality repairs will be useless if the thresholds are not protected from repeated corrosion. Here required steps after filling the holes:

1️⃣ Primer:

• 🟢 Acidic soil (for example, Reoflex) - applied as the first layer to protect the metal.
• 🟡 Epoxy primer (for example, Novol Protect 360) - creates a barrier against moisture.

2️⃣ Sealing seams:

• 🔧 Use seam sealant (for example, Terostat 922 or 3M 08693).
• 🎨Apply using sealant gun in an even layer.

3️⃣ Anti-corrosion mastic:

• 🛡️ For internal cavities: ML-mastic or Dinitrol 4010 (applied through technological holes).
• 🖌️ For the outer surface: bitumen mastic (for example, Kerrys).

4️⃣ Additional protection:

• 🔥 Waxing (for example, Turtle Wax) - for external surfaces.
• 🧲 Installation of mudguards — reduces the ingress of dirt onto the thresholds.

📅 Reprocessing schedule:

Type of protection	Refresh rate	Signs of wear
Internal ML mastic	Every 2–3 years	The appearance of rust at the joints
External bitumen mastic	Every year	Cracks, peeling
Seam sealant	At every body repair	Delamination, color change

⚠️ Attention: After anti-corrosion treatment, do not wash the car for 3-5 days - the protection needs time to polymerize. Also avoid high-pressure washing in the sill area - the jet of water can damage the fresh layer of mastic.

Common mistakes when repairing thresholds and how to avoid them

Even experienced car owners make mistakes that ruin all repair efforts. Here are the most common:

🔴 Mistake 1: Repair “with live bait” without removing rust.

✅ How to correctly: Remove rust before bare metal - even a small focus of corrosion will continue to destroy the threshold from the inside. Use sandblasting or drill with brush attachment for hard to reach places.

🔴 Mistake 2: Using regular putty for large holes.

✅ How to correctly: For holes larger than 1 cm, use fiberglass putty or combine it with epoxy resin.

🔴 Mistake 3: Painting without a primer.

✅ How to correctly: Soil layer required — it improves paint adhesion and protects the metal. Suitable for thresholds epoxy primer (for example, Novol 420).

🔴 Mistake 4: Ignoring internal anti-corrosion treatment.

✅ How to correctly: After repair, treat the threshold from the inside ML-mastic or Dinitrol through technological holes.

🔴 Error 5: Welding without disconnecting the battery.

✅ How to correctly: Always disconnect the battery terminals before welding to avoid damage to the electronics. On modern cars (for example, Skoda Octavia or Hyundai Solaris) it is also recommended to disable body electronics control unit.

🔴 Mistake 6: Using incompatible metals.

✅ How to correctly: Do not combine steel and aluminum without insulating gasket - this will lead to electrochemical corrosion. For aluminum thresholds, use aluminum rivets and argon welding.

🔴 Mistake 7: Painting in a cold garage.

✅ How to correctly: The temperature during painting should be 18–25°C, humidity - no higher than 60%. Otherwise, the paint will lie unevenly or bubble.

FAQ: Answers to frequently asked questions

Is it possible to repair a hole in the threshold without welding?

Yes, for holes up to 5 cm, alternative methods are suitable:

• 🔧 Rivets + metal patch - for holes 3–10 cm.
• 🧴 Epoxy resin + fiberglass - for holes 1–5 cm.
• 🎨 Fiberglass putty - only for small holes (up to 1.5 cm).

However, remember: without welding you will not restore initial strength threshold. These methods are suitable for temporary repairs or when welding is not possible (for example on aluminum thresholds without argon welding).

How long will a threshold repair last without welding?

The service life depends on the method and operating conditions:

• 🔹 Putty: 6 months – 1.5 years (cracks from vibrations).
• 🔹 Epoxy resin + fiberglass: 2–4 years (if not damaged mechanically).
• 🔹 Rivets + metal patch: 3–7 years (with proper anti-corrosion treatment).

To extend service life:

• 🧴 Update regularly anti-corrosion mastic (once every 1–2 years).
• 🚿 Avoid high pressure washing in the threshold area.
• 🔧 Check the condition of the repair every spring after winter.

What is the difference between repairing thresholds on a galvanized body?

Galvanized thresholds (for example, on Volkswagen Passat B6 or Ford Focus 3) is more difficult to repair due to the risk of damage to the zinc layer. Features:

• 🔧 Welding: use spot welding or MIG with minimum currentso as not to burn the zinc. After welding, be sure to treat the seam zinc-containing primer (for example, Zinc Rich Primer).
• 🧴 Putty: Before applying, strip the zinc layer down to metal and apply adhesive primer (for example <