Repairing leaky car sills: from preparation to painting

Car sills are one of the most vulnerable parts of the body. Constant exposure to moisture, reagents and mechanical damage leads to corrosion, which over time turns into through holes. If you ignore the problem, rust will spread to the side members and load-bearing elements, which will make repairs much more expensive. Fortunately, repair leaky sills you can do it yourself - without going to a car service center. The main thing is to choose the right materials and follow the technology.

In this article we will analyze all the stages: from diagnosing damage to final painting. You will learn what tools you will need, how to prepare the surface, what is the best way to seal holes (with or without welding), and how to protect thresholds from repeated corrosion. We will pay special attention to typical mistakes that beginners make - this will help save time and money.

If you have never done body repair, don’t worry: step-by-step instructions with photos and videos (links are provided at the end of the article) will make the process clear even for beginners. And for those who already have experience, we have prepared professional life hacks, which will speed up work and improve results.

1. Diagnosis of damage: when the thresholds are already full of holes, and when they can still be saved

Before you take up the tools, you need to assess the scale of the problem. Through holes are an already advanced stage of corrosion, but often under the rusty top layer the metal still retains strength. Here's how to determine exactly what your car needs:

Signs of a critical state of thresholds:

• 🔍 Visible through holes (even small ones - up to 1 cm).
• 🚗 Crunching or sinking of metal when pressed with a foot.
• 🔧 Rust on the inside of the threshold (visible when the interior trim is removed).
• 💨 Air whistling at speed from under the doors.

If the holes are small (up to 3–5 cm), they can be patched or used epoxy putty with fiberglass. In case of extensive damage (more than 30% of the threshold area), you will have to cut out the area and weld in new metal. In the most severe cases - when rust has eaten the side members - a complete replacement of the threshold will be required.

⚠️ Attention: If during inspection you find that rust has eaten through the metal around the jack fastenings or the welding points of the side members - contact a car service immediately. Self-repair in this case can weaken the rigidity of the body and make the car dangerous for use.

For an accurate diagnosis, use:

• 🔦 Flashlight and mirror on a flexible handle (for inspecting internal cavities).
• 🧲 Magnet (apply to the threshold - if it doesn’t stick, the metal has already become thinner).
• 📏 Caliper or ruler (to measure the thickness of the metal).

2. Tools and materials: what you will need for repairs

The list of tools depends on the chosen repair method (welding or without), but there is a basic set that will be useful in any case. Don’t skimp on materials - cheap putty or primer will quickly peel off, and the work will have to be redone.

Required tool:

• 🔨 Angle grinder (grinder) with cutting and cleaning wheels.
• 🔧 Drill with a brush attachment for removing rust.
• 🧲 Welding machine (if you plan to weld patches).
• 🖌️ Spatulas (metal and rubber).
• 🎨 Spray gun or spray paint.

Consumables:

Material	Purpose	Recommended Brands
Epoxy putty with fiberglass	Fixing small holes without welding	3M, Novol, Body
Sheet metal (0.8–1 mm)	Welding patches	Galvanized steel or aluminum
Anti-corrosion primer	Protecting metal before painting	Reoflex, Dinitrol, Tectyl
Car paint	Final coating	Selection by color code of your car
Rust converter	Treatment before puttying	Tsinkar, Kudo

If you plan to cook rapids, additionally prepare:

• 🔥 Auto-darkening welding mask.
• 🧤 Welding gloves (leather or canvas).
• 🧲 Clips or clamps for fixing patches.
• 🔧 Shielding gas (carbon dioxide or argon) for a semi-automatic machine.

What can replace a welding machine?

If you do not have a welding machine, small holes (up to 2–3 cm) can be repaired using:

- Epoxy resin with reinforcing mesh (sold in auto stores).

- Special repair pads (for example, from 3M or Permatex).

- Cold welding (suitable for temporary repairs).

However, remember: such methods are less durable than welding and require more thorough anti-corrosion treatment.

3. Preparing the threshold for repair: removing rust and old treatment

80% of success depends on the quality of preparation. If you leave even small pockets of rust or do not clean the metal well, corrosion will quickly return. It is more convenient to work on a lift or overpass, but you can get by with jacks (be sure to secure the car with stops!).

Step by step instructions:

1. Remove the interior trim (if there is rust inside the threshold). On most cars it is attached with clips or screws.
2. Remove old putty and paint grinder with a grinding wheel (grain 40–80). Work carefully so as not to thin the metal.
3. Cut out rusty metal to the "live" layer. If the hole is through, widen it to make it easier to weld or putty.
4. Finish the edges file or sandpaper (P80–P120). They should be smooth, without burrs.
5. Apply rust converter (for example, Tsinkar) for 15–20 minutes, then rinse with water and dry.

⚠️ Attention: When working with an angle grinder, use respirator and glasses — dust from rust and paint is toxic. If the thresholds are treated with mastic, remove it with a hair dryer and not with open fire (risk of fire!).

For convenience, you can do patch template from cardboard, applying it to the hole. This will help you cut the metal patch accurately and avoid gaps when welding.

Remove interior trim (if necessary)

Strip away rust to bare metal

Cut out damaged areas (if there are holes)

File the edges

Apply rust converter and rinse

Dry the surface (with a hairdryer or naturally) -->

4. Repair methods: welding vs. putty

The method you choose depends on the size of the holes, your skills, and the tools available. Welding is more reliable, but requires experience, and putty is simpler, but less durable. Let's consider both options in detail.

Welding patches (for holes 1 cm and larger):

• 🔧 Cut out a patch from sheet metal (0.8–1 mm thick) according to the template.
• 🔥 Tack it at 2-3 points by welding, then weld the seam with a solid line.
• 🔨 Clean the seam with a grinder, removing metal deposits.
• 🖌️ Apply putty to the joints to level the surface.

For welding use semi-automatic in carbon dioxide environment — it gives minimal warping of the metal. If you have little experience, practice on unnecessary pieces of tin.

Putty without welding (for holes up to 3–5 cm):

• 🧴 Use epoxy putty with fiberglass (for example, 3M 05887).
• 🧶 For reinforcement, place fiberglass or metal mesh on top.
• 🕒 Let each layer dry (usually 15–20 minutes at +20°C).
• 📏 After drying, sand the surface (P120–P240).

⚠️ Attention: Putty without welding is a temporary solution! After 1-2 years, rust may appear underneath again. For durability, combine methods: weld patches, and fill small uneven areas with putty.

If the hole is located in lower part of the threshold (where moisture accumulates), be sure to use welding. The putty in this area will quickly peel off due to constant dampness.

5. Anti-corrosion treatment and primer

After repairing the metal, you need to protect it from new corrosion. Many people skip this stage, but it is what determines how long your repair will last - 1 year or 10 years.

Step by step processing:

1. Degrease the surface acetone or White spirit.
2. Apply acid primer (for example, Reoflex Acid Primer) - it creates a protective film and improves adhesion.
3. Cover with acrylic primer in 2–3 layers with interlayer drying for 10–15 minutes.
4. Treat the internal cavity threshold Movilem or Dinitrol through technological holes.

For internal processing use flexible tube sprayer - it allows you to get to the most secluded places. Do not skimp on the anticorrosive agent: it is better to apply it in excess than to leave untreated areas.

Tips for choosing soil:

• 🟢 Epoxy primer - the most durable, but difficult to apply (requires a perfectly dry surface).
• 🟡 Acrylic primer - universal, suitable for beginners.
• 🔴 Acidic soil - only for metal, cannot be applied to putty!

⚠️ Attention: If you have repaired thresholds galvanized car (for example, Volkswagen, Skoda, Hyundai after 2010), use primer with zinc (for example, Zinc Primer from Body). It will restore the protective layer and prevent electrochemical corrosion.

6. Painting and final protection

The last stage is painting and protection from external influences. It is important to take your time here: each layer must dry, otherwise the paint will bubble.

Painting instructions:

1. Sand the soil sandpaper (P320–P400) until matte.
2. Degrease the surface lint-free cloth soaked in Antisilicone.
3. Apply a base coat of paint (2-3 layers with 10 minutes drying in between).
4. Varnish (if a two-component system is used).
5. Polish after 24 hours with a soft paste (for example, 3M 09374).

Suitable for painting small areas aerosol cans with adapter for compressor. If you are painting the entire threshold, it is better to rent a spray gun with a nozzle 1.3–1.4 mm.

Additional protection:

• 🛡️Apply anti-gravel film to the lower part of the threshold (for example, 3M Scotchgard).
• 🧴 Treat the seams body sealant (for example, Teroson).
• 🔧 Install plastic linings (if they were on your car model).

After painting, avoid washing the car for 3-5 days (depending on the type of paint). If used acrylic varnish, complete polymerization takes up to 2 weeks.

7. Common mistakes and how to avoid them

Even experienced craftsmen sometimes make mistakes that shorten the service life of repaired thresholds. Here are the most common mistakes and ways to prevent them:

Error 1: Incomplete rust removal

• 🔴 Consequences: Corrosion continues to spread under the putty.
• ✅ Solution: Use rust converter and check the metal with a magnet (it should not “bounce”).

Mistake 2: Welding without protecting adjacent parts

• 🔴 Consequences: Paint burns on a door or fender.
• ✅ Solution: Close adjacent panels asbestos sheet or liquid mask (for example, Body Guard).

Mistake 3: Thick layer of putty

• 🔴 Consequences: Cracks and peeling after 6–12 months.
• ✅ Solution: Apply putty in layers no thicker than 2–3 mm, allowing each layer to dry.

Mistake 4: Ignoring internal anti-corrosion treatment

• 🔴 Consequences: Rust from the inside destroys the threshold, despite external repairs.
• ✅ Solution: Fill into the threshold cavity Movil or Tektil through drainage holes.

Mistake 5: Painting without a primer

• 🔴 Consequences: The paint is peeling, the metal is rusting.
• ✅ Solution: Always use acid + acrylic primer before painting.

If you doubt your abilities, practice on an unnecessary part (for example, an old door or hood). This will help avoid mistakes during basic repairs.

8. Cost of repairs: what is cheaper - do it yourself or send it to a service center?

The cost of repairing thresholds depends on the extent of damage, restoration method and region. The average prices in Russia are as follows:

Type of work	Cost of service (RUB)	Cost yourself (RUB)
Local repair (hole up to 5 cm, putty)	3 000–6 000	800–1 500
Welding patches (1–2 holes)	5 000–10 000	1 500–3 000
Complete replacement of the threshold (welding of a new part)	15 000–30 000	5 000–8 000
Anti-corrosion treatment (internal + external)	2 000–4 000	500–1 200
Painting the threshold (with primer)	4 000–8 000	1 000–2 500

As you can see, DIY repairs are 3–5 times cheaper. However, keep in mind:

• ⏳ Time: The service will do the work in 1-2 days, but it may take a week to do it yourself.
• 🛠️ Tool: If you don’t have an angle grinder or a welding machine, buying/renting them will eat up some of the savings.
• 🎨 Quality: Professional painting in a booth looks better than spray painting in a garage.

⚠️ Attention: If you are repairing sills on a car older than 10 years, evaluate the feasibility of the investment. For example, for VAZ 2109 1998 replacement of thresholds for 30,000 rubles. may not be profitable - it is cheaper to find a car in better condition.

FAQ: Frequently asked questions about repairing thresholds

Is it possible to repair holes in thresholds without welding?

Yes, but this is a temporary solution. Suitable for holes up to 3–5 cm epoxy putty with fiberglass (for example, 3M 05887) or repair pads from Permatex. However, without welding, the risk of repeated corrosion is higher - it is recommended to combine methods: weld large holes and seal small ones with putty.

For temporary repairs (for 1–2 years) can be used cold welding (for example, Poxipol), but it cannot withstand heavy loads.

What metal is best to use for patches?

Optimal choice - galvanized steel 0.8–1 mm thick. It is resistant to corrosion and easy to cook. For foreign cars (for example, Toyota, Honda) it's better to take aluminum, but it is more difficult to weld - you will need argon and special wire.

Alternative - repair inserts from the manufacturer (for example, for VAZ or GAZelle). They fit perfectly and make work easy.

How to treat the thresholds from the inside after repair?

For internal processing use:

• Liquid anticorrosives (Movil, Tektil, Dinitrol) - applied by spray through drainage holes.
• Paraffin compounds (Noxudol, Waxoyl) - create an elastic protective film.
• Zinc sprays (Zinc Spray) - for additional protection of welds.

Before processing, dry the threshold with a hair dryer - moisture inside will accelerate corrosion.

How long does it take for putty to dry on thresholds?

Drying time depends on the type of putty and temperature:

• Epoxy putty (for example, 3M) - 15–20 minutes at +20°C.
• Polyester putty (for example, Novol) - 5-10 minutes, but requires application in 2-3 layers.
• Putty with aluminum powder - up to 1 hour (used for deep dents).

You can speed up drying with a hairdryer, but do not overheat - the putty may crack. Optimal temperature for drying: +18–25°C.

How to paint thresholds so that repairs are not visible?

To make the repair invisible:

1. Use paint of the same code, as on the car (indicated in the registration certificate or on a plate under the hood).
2. Apply paint in 3 thin layers with interlayer drying for 10–15 minutes.
3. After painting, polish the threshold abrasive paste (3M 09374) to even out the shine.
4. If the sills are matte (for example, on crossovers), use matte varnish.

For perfect results, color the entire threshold, and not just the area being repaired - this will hide color transitions.