The thresholds of a car are one of the most vulnerable areas of the body, which is the first to take the blow from road reagents, sand and moisture. According to statistics NACE (International Corrosion Control Association), up to 40% body damage begin precisely from thresholds, if they are not protected systematically. At the same time 9 out of 10 owners of cars over 5 years old experience rust on their sills due to improper or lack of treatment. In this article, we’ll look at which formulations really work and which ones only mask the problem, and we’ll also show you how to apply them without mistakes.
Many car owners mistakenly believe that it is enough to cover the sills with mastic once and you can forget about corrosion for years. In practice, even the most expensive anticorrosion agents require regular updates (once every 1–2 years), and the wrong choice of product can accelerate the destruction of the metal. For example, water-based bitumen mastics often crack at sub-zero temperatures, and some “wax” sprays are washed off after the first wash. We tested 12 popular products and selected those that really protect, taking into account the climatic conditions of Russia.
Why do sills rust faster than other body parts?
The rapids are located at zone of maximum impact aggressive factors:
- 💦 Moisture and reagents: when driving, splashes from the wheels fall directly onto the lower part of the sills, where salt, sand and chemical mixtures from the roads accumulate.
- 🪨 Mechanical damage: stones, curbs and car wash brushes scratch the protective coating, exposing the metal.
- 🔥 Temperature changes: in winter, the thresholds heat up from the engine and cool sharply in the cold, which leads to microcracks in the paintwork.
- 🔋 Galvanic couples: If the thresholds come into contact with aluminum or galvanized parts (for example, wheel arch liners), galvanic corrosion occurs.
In addition, on many budget models (for example, Renault Logan, Lada Granta, Hyundai Solaris) sills are made of thinner metal than the hood or fenders, and factory galvanization is often insufficient. As a result, after 3–4 years, even with careful use, "saffron milk caps" — the first signs of through corrosion.
Top 7 means for treating thresholds outside: comparison and prices
All anti-corrosion compounds for thresholds are divided into three groups:
- Mastics (bitumen, rubber, polymer) - create a thick protective layer, but require preliminary surface preparation.
- Liquid fender liners (rubber, PVC) - elastic coatings that do not crack due to vibration.
- Wax and polymer sprays — temporary protection (6–12 months), suitable for prevention.
| Means | Type | Protection period | Price for 1 l, ₽ | Pros | Cons |
|---|---|---|---|---|---|
| Dinitrol 479 | Rubber mastic | 3–5 years | 1 200 | Does not crack in the cold, is resistant to detergents | Difficult to apply without a sprayer |
| Tectyl Bodysafe | Wax spray | 1–2 years | 850 | Dries quickly, no preparation required | Washable after 5-6 washes |
| Liqui Moly Unterbodenschutz | Bitumen-rubber mastic | 4–6 years | 1 500 | High adhesion, protects against salt | Takes a long time to dry (24 hours) |
| Krown T40 | Oil anticorrosive | 2–3 years | 900 | Penetrates rust, does not require complete cleaning | Attracts dust and gets your hands dirty |
| 3M Undercoating | Rubber coating | 5+ years | 2 100 | Impact resistant, not afraid of chemicals | High price, requires professional application |
For most passenger cars, the optimal choice will be Dinitrol 479 or Liqui Moly — they combine durability and affordability. If the thresholds are already affected by rust, it is better to use Krown T40which stops corrosion. Suitable for temporary protection (for example, before winter) Tectyl Bodysafe.
Before purchasing, check the product’s compatibility with your car’s paintwork! Some mastics (for example, solvent-based) can corrode the paint on plastic door sill trims.
Step-by-step instructions: how to properly process thresholds
Even the most expensive product will not work if the surface is not prepared correctly. Let's look at the process using mastic as an example Dinitrol 479:
Washing sills with car shampoo (remove dirt and grease)|Treatment with a wire brush (to remove rust)|Degreasing with solvent (for example, White spirit)|Sealing adjacent surfaces with masking tape|Hair drying (temperature no higher than 60°C)-->
Step 1. Washing and drying
Use a touchless washer with a pressure of at least 100 barto knock dirt out of the cracks. Pay special attention to the junctions of the thresholds with the body - this is where the most reagents accumulate. After washing, dry the sills with a hair dryer or leave the car in the sun for 2–3 hours.
Step 2: Removing rust
For small areas of corrosion, use rust converter (for example, Tsinkar), for deep damage - sandpaper P80–P120 or a metal brush. Important to remove all the loose rust, otherwise the mastic will not stick.
⚠️ Attention: Do not use an angle grinder to clean thresholds! Thin metal is easy to burn, and sparks can damage the paintwork of adjacent parts.
Step 3. Applying mastic
Apply the mastic to 2–3 layers at intervals of 15–20 minutes. Optimal operating temperature: +15…+25°C. For even distribution, use brush with stiff bristles or pneumatic sprayer. The layer thickness should be 0.5–1 mm - if less, the protection will be ineffective.
Step 4. Drying and polymerization
After application, leave the vehicle in a dry place for 24 hours (for bitumen mastics) or 12 o'clock (for rubber). Do not wash or operate the machine in the rain for 3–5 days - this time is needed for complete polymerization.
The main mistake when processing thresholds is saving on preparation. Even the most expensive mastic will not work if the rust is not removed and the surface is not degreased!
What not to do when processing thresholds
Some “folk” methods not only do not help, but also accelerate corrosion. Here top 5 mistakes, which allow 80% of car owners:
- 🧴 Applying mastic to wet thresholds: water under the anticorrosive layer leads to under-film corrosion — rust eats away metal from the inside, remaining invisible.
- 🔥 Using a hair dryer for drying: temperature higher
80°Cdestroys the structure of the mastic, making it fragile. - 🧽 Washing thresholds with Karcher after treatment: a high-pressure jet tears off fresh anticorrosive. Carry out the first wash no earlier than a week later.
- 🎨 Painting thresholds over mastic: paint does not stick to bitumen compounds. If you want to paint, use special primers (for example, App Anti-Rust Primer).
- ❄️ Processing at temperatures below +5°C: Most mastics do not polymerize in the cold and remain sticky.
⚠️ Attention: Never mix different types of anticorrosives! For example, if you applied oil Krown, and covered with bitumen mastic on top, the layers will not adhere, and the protection will be ineffective.
How to care for thresholds after treatment
Even after high-quality processing, thresholds require regular care, especially in winter. Here's what to do:
- 🚿 Wash once every 2 weeks: use gentle shampoos without abrasives (for example, Karcher RM 539). Avoid hard brushes!
- 🛡️ Applying wax spray: treat the thresholds once every 3 months Tectyl Bodysafe or Sonax Underbody Protection for added protection.
- 🔍 Inspection on the lift: check the thresholds every six months for chips or peeling mastic. If damage is found, touch up the paint immediately.
- 🚗 Avoid parking on the grass: Moisture from dew accelerates corrosion from below. It is better to leave the car on asphalt or concrete.
If you live in an area with aggressive winters (for example, Moscow, St. Petersburg, Novosibirsk), recommended additional processing of wheel arches - this will reduce the amount of splashes falling on the thresholds. Suitable for this Liqui Moly Unterbodenschutz or 3M Undercoating.
What to do if the mastic begins to peel off?
If 1-2 years after treatment you notice that the mastic is cracking or coming off, do not rush to apply a new layer on top of the old one! First, remove the exfoliated areas with a spatula or a metal brush, then degrease the surface and apply fresh anticorrosive. If you ignore the peelings, moisture will begin to accumulate underneath them, and rust will corrode the metal from the inside.
Comparison of professional and self-processing
Many car owners doubt whether it is worth paying for processing at a service center or whether they can handle it on their own. Let's look at the pros and cons of each option:
| Criterion | Self-processing | Professional processing |
|---|---|---|
| Cost | 1,500–3,000 ₽ (materials) | 5,000–15,000 ₽ (depending on the car) |
| Quality of preparation | Depends on skill (risk of missing rust) | Sandblasting, complete removal of corrosion |
| Service life | 2–3 years (if applied correctly) | 4–6 years (service warranty) |
| Time | 1–2 days (including drying) | 3–5 hours (without your participation) |
| Additional services | — | Treatment of hidden cavities, anti-gravel coating |
Self-processing is justified if:
- Do you have experience working with anticorrosion agents?
- The thresholds are in good condition (no through corrosion).
- You're willing to spend time preparing and drying.
You should contact the service if:
- The rust has already eaten right through the metal.
- It is necessary to process hidden cavities (spars, amplifiers).
- You want a job guarantee.
When choosing a service, ask what kind of mastic they use. Some service stations save money and apply cheap bitumen compounds that crack after a year. True professionals work with Dinitrol, Tectyl or 3M.
Frequently asked questions about threshold processing
Is it possible to treat thresholds in winter?
Technically it is possible, but the efficiency will be lower. The optimal temperature for applying most mastics is +15…+25°C. At sub-zero temperatures, the compounds thicken and do not penetrate microcracks well. If you have to process in winter, use solvent-based mastics (for example, Krown T40) and work in a heated garage.
What color of mastic should I choose: black or gray?
Color does not affect the protective properties, but there are nuances:
- Black mastic It masks rust and chips better, but gets hotter in the sun.
- Gray mastic less noticeable on light-colored cars and does not attract heat as much.
Black color is suitable for most cars - it is universal and retains its appearance longer.
How to treat plastic door sill trims?
Plastic linings do not rust, but can crack and fade. To protect them use:
- Wax polishes (for example, Sonax Plastik-Pflege) - restore color and protect against UV rays.
- Rubber conditioners (for example, Gummi-Pflege) - prevent cracking.
Do not apply bitumen mastics to plastic - they leave permanent stains!
How many layers of mastic should I apply?
Optimal quantity - 2–3 layers:
- 1st layer — thin, penetrating (fills microcracks).
- 2nd layer - main, thickness
0.5–1 mm. - 3rd layer - additional, applied to problem areas (joints, welds).
It is not advisable to apply more than three layers - the mastic will take a long time to dry and may crack.
Is it possible to paint the thresholds after treating them with mastic?
Yes, but only after complete polymerization of the mastic (through 3–5 days). Use:
- Acrylic paint in cans (for example, Motip).
- Primer for plastic (if you paint the overlays).
Before painting, be sure to degrease the surface Antisilicon and apply a layer of primer.