Car thresholds are one of the most vulnerable areas of the body, which are the first to suffer from corrosion, chips and mechanical damage. Even small scratches on metal can turn into spots of rust over time if they are not treated promptly. But how to cover the car sills so that the protection is durable and the result is aesthetically pleasing? The choice of product depends on the degree of damage, the material of the threshold (steel, aluminum, plastic) and even the climatic conditions of the car’s operation.
Many drivers make a critical mistake: they mask rust with cosmetics (for example, car enamel) without first cleaning and treating the metal. Such “treatment” only accelerates the destruction - corrosion continues to spread under the paint layer. In this article we will analyze professional methods of threshold protection, from budget solutions to premium anti-corrosion compounds, and we will also give step-by-step instructions, taking into account typical mistakes.
If you notice paint bubbles, red spots or even through holes on the thresholds, do not put off repairs. The sooner you start processing, the cheaper and easier it will be to restore the metal. And for those who plan to sell a car, high-quality threshold protection can increase its market value by 5–15% (according to appraisers Autocheck).
Why do sills rust faster than other body parts?
Thresholds are exposed to the aggressive influence of several factors at once:
- Mechanical damage: Sand, gravel and road salt will scratch the paintwork, exposing the metal.
- Humidity: Water collects in drainage holes and joints, creating ideal conditions for corrosion.
- Chemical reagents: winter deicing mixtures accelerate metal oxidation by 3–5 times (according to NACE International).
- Design features: on many models (eg VAZ 2110 or Renault Logan) thresholds have cavities where condensation accumulates.
Interesting fact: on cars with galvanized body (for example, Volkswagen Golf or Skoda Octavia) corrosion of thresholds appears 2–3 times less often. However, even they are not immune to rust in places where they are chipped or after unqualified repairs.
According to statistics from service centers, up to 70% of body work to eliminate corrosion occurs on the sills and wheel arches. At the same time the average cost of restoring one threshold at a car service is from 8,000 to 25,000 rubles, depending on the degree of damage.
Top 5 threshold treatment products: comparison and recommendations
The choice of means depends on the task:
- 🔧 Minor scratches no rust → an anti-gravel composition or a wax protective layer is sufficient.
- 🛠️ Surface corrosion → needs cleaning + rust converter + mastic.
- 🚗 Through holes or deep rust → welding/putty + full anti-corrosion treatment required.
The table below compares the most effective products, taking into account their cost, durability and complexity of application:
| Means | Type | Protection period | Price (per 1 l) | Features |
|---|---|---|---|---|
| Mastic Bitumast | Bitumen mastic | 3–5 years | 350–500 ₽ | Good for the lower part of the rapids, but does not tolerate high temperatures. |
| Dinitrol 479 | Anti-corrosion wax | 2–3 years | 1 200–1 500 ₽ | Penetrates microcracks, but requires annual renewal. |
| Body 930 | Anti-gravel coating | 1–2 years | 600–800 ₽ | Ideal for chip protection, but not suitable for rust. |
| Rust Stop | Rust converter | — | 400–600 ₽ | Transforms rust into a protective layer, but requires subsequent treatment with mastic. |
| 3M Undercoating | Professional anticorrosive | 5–7 years | 2 500–3 000 ₽ | The most durable solution, but difficult to apply without experience. |
For most drivers, the best option is a combination rust converter (for example, Rust Stop) + bitumen mastic (Mastic Bitumast). This combination provides reliable protection for 3–4 years at minimal cost.
⚠️ Attention: Do not use products based on thresholds. silicone (for example, WD-40) - they do not protect against corrosion, but only temporarily mask it. Silicone sprays attract dust and impair paint adhesion.
Step-by-step instructions: how to cover thresholds with your own hands
The thresholding process can be divided into 5 key stages. If you miss even one of them, the result will be short-lived.
Remove dirt and oil from surface|Strip rust to bare metal|Treat with rust converter|Apply primer (if necessary)|Protect adjacent elements with masking tape-->
Step 1: Cleaning and degreasing
Use car shampoo (eg. Karcher>) and a stiff bristle brush to remove dirt from all crevices. Suitable for degreasing White spirit or a specialized degreaser (APP W900). Never use gasoline or kerosene - they leave an oil film that impairs the adhesion of the mastic to the metal.
Step 2: Removing rust
For small areas of corrosion, sandpaper is sufficient. P80–P120. If the rust is deep, use a wire brush or drill with a brush attachment. Important to remove all rust, even if it is hidden under a layer of paint.
Step 3: Apply Rust Converter
After stripping, apply converter (e.g. Tsinkar or Rust Stop) with a brush or spray. Let it dry for 15-20 minutes - during this time the rust will turn into a gray protective layer. Then rinse off the residue with water and dry the surface.
Step 4: Primer (optional)
If the threshold is severely damaged, apply 1-2 layers of anti-corrosion primer (Body 960). This will improve the adhesion of the mastic and extend the service life of the coating.
Step 5. Applying a protective composition
For bitumen mastic, use a brush or spatula; for liquid anticorrosives (for example, Dinitrol) - sprayer. Apply the product in 2-3 layers at intervals of 10-15 minutes. Pay special attention to:
- 🔹 Joints and welds
- 🔹 Drainage holes (they must not be blocked!)
- 🔹 Lower edge of the threshold
To apply the mastic evenly, heat the jar in warm water (up to 40–50°C) - this will make the composition more flexible and lay down in an even layer.
Common mistakes when processing thresholds and how to avoid them
Even experienced car owners sometimes make mistakes that ruin all their efforts. Here are the most common of them:
1. Poor rust removal
If you leave even a small area of corrosion under a layer of mastic, it will continue to spread. Use magnifying glass or a flashlight to check the quality of the stripping.
2. Ignoring drainage holes
Many people cover the holes with mastic, which causes moisture to accumulate inside the threshold. Correct: clean the holes with wire and treat them separately with anticorrosive agent (ML oil).
3. Applying mastic to a wet surface
Moisture under the protective layer will lead to swelling and peeling. Before processing, dry the threshold with a hair dryer or leave the car in the sun for 1–2 hours.
4. Saving on money
Cheap mastics (for example, "Avtoantikor" for 200 ₽) often contain solvents that attack the metal. It’s better to buy a high-quality composition once than to redo the work a year later.
⚠️ Attention: If there are already through holes in the threshold, mastic will not help - welding or installation of repair pads is required. Trying to fill the hole with putty will lead to peeling and accelerated corrosion.
What to do if rust appears again after a few months?
If rust returns after treatment, this means that:
1) You did not remove the entire corrosion area when cleaning.
2) Used low-quality mastic or applied it to a damp surface.
3) The internal cavity of the threshold (if there is one) was not treated.
In this case, you will have to repeat the procedure, but with complete disassembly of the threshold (if necessary) and processing ML oil or wax anticorrosive from the inside.
How to protect thresholds from corrosion for a long time: prevention
Even after high-quality treatment, thresholds require regular maintenance. Here's what the experts recommend:
- 🚿 Wash once every 2 weeks with mandatory cleaning of the thresholds from dirt and salt. Use contactless cleaning products (for example, Sonax Xtreme), so as not to scratch the coating.
- 🛡️ Applying protective wax every 3 months. Will do Turtle Wax Ice or Collinite 845.
- 🔧 Checking the Drain Holes before winter. If they are clogged, clear them with wire or compressed air.
- 🚗 Installation of mudguards (if they don't exist). They will reduce the ingress of sand and reagents onto the thresholds.
For owners of cars older than 5 years, it is especially important processing of internal cavities of thresholds. This can be done independently through technological holes (if any) or in the service using professional equipment (for example, pneumatic sprayer).
The cost of complete anti-corrosion treatment of thresholds in the service ranges from 3,000 to 10,000 rubles, but this investment pays off by extending the service life of the body. For example, on Toyota Corolla 2015, high-quality treatment can delay major repairs of thresholds for 5–7 years.
Comparison of methods: which is better - mastic, anti-gravel or liquid anti-corrosive?
Each method has its pros and cons. Let's figure out which one is suitable for your case:
| Method | Pros | Cons | Better for... |
|---|---|---|---|
| Bitumen mastic | Durable (3–5 years), resistant to mechanical damage | Difficult to apply, afraid of high temperatures | The lower part of the sills, wheel arches |
| Anti-gravel | Easy to apply and protects against chipping | Short lifespan (1-2 years), not suitable for rust | New thresholds without damage |
| Liquid anticorrosive (ML-oil) | Penetrates microcracks and protects from the inside | Requires re-treatment every 1–2 years | Internal cavities of thresholds |
| Wax compounds | Easy to renew, adds shine | Washable after 2-3 washes | External surface of thresholds (in addition to mastic) |
For maximum protection, experts recommend combined approach:
- Process the internal cavity of the threshold ML oil.
- Apply to the bottom bitumen mastic.
- Coat the outer surface anti-gravel or wax composition.
If your car is operated in conditions of high humidity (for example, in St. Petersburg or Sochi), give preference paraffin anticorrosives (Tectyl or Noxudol) - they repel water better than bitumen mastics.
When is it worth contacting the service, and when can you handle it yourself?
Self-processing of thresholds is justified if:
- 🔧 Surface damage (scratches, small rust spots).
- 🚗 You have the time and desire to understand the process.
- 💰 Budget is limited (independent processing will cost 3-5 times cheaper than the service).
You need to contact a car service if:
- 🔥 There are through holes or severe corrosion (welding required).
- 🔍 Thresholds have a complex design with internal cavities (for example, on Mercedes-Benz W211 or BMW E60).
- 🎨 The perfect appearance is important to you (the service will paint it to match the body color).
Average cost of service:
- Cleaning and processing the threshold without welding - 3 000–6 000 ₽.
- Repair of through corrosion (welding + putty) — 8 000–15 000 ₽.
- Full anti-corrosion treatment (including internal cavities) - 15 000–25 000 ₽.
If you decide to contact the service, choose proven stations with a guarantee for the work. Pay attention to reviews about the quality of anti-corrosion treatment - some services save on materials by using cheap analogues.
FAQ: Frequently asked questions about threshold protection
Is it possible to cover the thresholds with regular paint?
No, ordinary car enamel does not protect against corrosion - it only masks it. Paint can be applied only on top of anti-corrosion primer or mastic, otherwise rust will continue to spread under the layer.
How often do you need to update the anticorrosive on thresholds?
It depends on the product:
- Bitumen mastic - once every 3–4 years.
- Liquid anticorrosive agent (ML-oil) - once every 1–2 years.
- Wax compositions - every 3 months.
If you live in a region with salty winters (for example, Moscow, Yekaterinburg), reduce the interval by 30%.
How to treat the thresholds from the inside?
Suitable for internal cavities:
- ML oil (the most popular solution).
- Wax anticorrosives (Tectyl, Noxudol).
- Paraffin compounds (Dinitrol 1000).
They can be applied through technological holes or by spraying (in the service).
Is it possible to drive with rusty sills?
Technically yes, but it's dangerous:
- Rust weakens the body structure, which can lead to threshold deformation during an accident.
- On TO a car with severe corrosion may not be allowed through (according to
GOST R 51709-2001, through rust is a reason for refusal in the diagnostic card). - When selling a car, rusty sills will reduce its value by 10–30%.
Which sills rust more often: front or rear?
Service statistics show that front sills rust 1.5–2 times more often than the rear ones. This is due to:
- Large amounts of splashes from the front wheels.
- Close to the engine (higher temperatures accelerate corrosion).
- Frequent contact with curbs when parking.
However, on vehicles with rear wheel drive (for example, BMW 5-series or Mercedes E-Class) the rear sills suffer almost as much due to splashes from the rear wheels.