The thresholds of a car are the most vulnerable part of the body after the arches. They are constantly attacked by salt, sand, moisture and mechanical damage. Without proper treatment, rust appears after 2-3 years, and through corrosion - after 5-7. But the problem is not only aesthetics: rotten sills reduce body rigidity by 15-20%, which affects safety in case of an accident.

In this article - 5 proven methods for treating thresholds outside, which are used by professionals at service stations and experienced car owners. We'll sort it out bitumen and synthetic based mastics, liquid plastic, zinc-containing sprays, wax compositions and Movil with rust converter. We will also tell you what errors lead to processing lasting less than a year.

Spoiler: bitumen mastic in cans (type Kerr or Body 930) provides the worst protection among all options - it cracks after 6-12 months of active use. If you want results for 3-5 years, read about alternative methods.

1. Preparation of thresholds: without this, processing is useless

90% of anti-corrosion treatment failures are due to poor preparation. Even the most expensive composition will not adhere to rust or grease stains.

What to do necessarily:

  • 🧹 Cleaning from dirt: pressure wash (minimum 100 bar) with car shampoo. Particular attention is paid to joints and drainage holes.
  • 🔧 Rust removal: mechanically (brush, drill attachment) or chemically (rust converter type Tsinkar).
  • 🧴 Degreasing: White spirit, Antisilicone or specialized degreasers (APP W900).
  • 🔥 Drying: hairdryer or in a warm garage (minimum 2 hours at +20°C). Surface moisture is the main enemy of adhesion.

If the thresholds already have holes or swollen paintwork, treatment from the outside will not help - you need patch welding or epoxy putty with reinforcement. Temporary solution - zinc spray with filler (for example, Zinc Rich Primer from Rust-Oleum), but this will delay repairs for a maximum of 1-2 seasons.

Remove dirt with a high pressure washer

Treat rust with a converter or mechanically

Degrease the surface with white spirit

Dry with a hairdryer or in a warm room

Use P80-P120 sandpaper to clean areas with peeling paintwork -->

⚠️ Attention: Never apply anticorrosive over Movilya or other oil compositions - they are not compatible with most mastics. If the thresholds have been previously treated, remove the old layer with solvent (Solvent 646).

2. Bitumen mastic: cheap, but short-lived

The most popular (and most controversial) method. Bitumen mastics type Body 930, Kerr or Tectyl They are inexpensive (from 300 rubles/liter), but have critical disadvantages:

  • 🕒 Service life: 1-2 seasons with active use (cracks due to temperature changes).
  • 🔧 Difficulty of application: requires heating (hairdryer or water bath) and even distribution with a brush.
  • 🚗 Appearance: Matte black surface that gets dirty and attracts dirt.

Where it is justified to use bitumen mastic:

  • ✅ Temporary protection for 1 season (for example, before selling a car).
  • ✅ Processing of internal cavities of thresholds (paired with Movil).
  • ✅ Budget repair of old cars (where appearance is not important).

Application technology:

  1. Heat the mastic to +60-70°C (in a water bath or a hair dryer).
  2. Apply with a brush or spatula in a layer of 1-2 mm.
  3. Dry for 24 hours (do not wash the car during this time!).
  4. If necessary, paint the top in the body color (for example, spray paint Motip).

Body 930

Kerr

Tectyl

Other brand

Haven't processed it yet -->

⚠️ Attention: Bitumen mastic is incompatible with liquid plastic and wax compounds. If you plan to switch to another type of protection in the future, you will have to completely remove the old layer.

3. Liquid plastic: protection + aesthetics

The best option for those who want keep the factory appearance of the thresholds and receive protection for 3-5 years. Liquid plastic (eg. Plasti Dip, Liquid Rubber or Raptor) forms an elastic coating that:

  • 🛡️ Protects against chips and scratches (layer thickness 0.5-1.5 mm).
  • 🎨 Retains factory color (can be tinted to match the body).
  • 🧽 Easy to clean and does not get dirty (unlike bitumen mastic).

Cons:

  • 💰 More expensive than mastic (from 1500 rubles/liter).
  • 🔧 Requires perfect preparation (any defects will be visible).
  • 🕒 Complete drying - 48 hours (the machine cannot be used).

Step by step instructions:

  1. Process thresholds antisilicon and dry.
  2. Apply 2-3 layers of primer (for example, Plasti Dip Primer) with interlayer drying for 15 minutes.
  3. Spray liquid plastic from a can or spray gun (distance 20-30 cm).
  4. Dry for 48 hours at +20°C (you can speed it up with an IR lamp).
How to remove liquid plastic if it is rubbed?

It can be removed mechanically (with a scraper) or chemically - using Plasti Dip Remover or White spirit. After removal, the thresholds must be prepared and processed again.

Parameter Bitumen mastic Liquid plastic
Protection period 1-2 years 3-5 years
Cost (per 1 liter) 300-600 rub. 1500-3000 rub.
Difficulty of application Medium (needs heating) High (requires experience)
Appearance Matte black Glossy/matte, tintable
Wash resistance Low (gets dirty) High

4. Zinc sprays: for rusty thresholds and chips

If there are already pockets of corrosion or chipped to metal, the best choice is zinc-containing sprays (for example, Zinc Rich Primer, Berner Zinc Spray or Liqui Moly Zink-Spray). They are:

  • 🛡️ Stop the rust due to electrochemical protection (zinc sacrifices itself instead of iron).
  • 🔧 Doesn't require perfect cleaning - Can be applied to light rust.
  • 🎨 Can be used as a base for painting (compatible with acrylic enamels).

How to apply:

  1. Clean off rust bare metal (using a drill attachment or manually).
  2. Degrease antisilicon.
  3. Apply 2-3 layers of zinc spray from a distance of 20-30 cm (drying between layers for 10 minutes).
  4. Paint on top if necessary spray paint (for example, Motip or Kudo).

Zinc sprays are the only option that cures rust, and does not just disguise it. But they do not replace a full-fledged anti-corrosion treatment: it is better to apply it on top liquid plastic or wax composition.

💡

To treat drainage holes in thresholds, use zinc spray with tube nozzle (for example, Würth Zinc Spray). This will prevent corrosion from the inside.

5. Wax compounds: for new cars and prevention

If the thresholds are in perfect condition (no rust, chips, swollen paintwork), the best prevention is wax anticorrosion agents type Turtle Wax, Sonax or Collinite 845. They are:

  • 🛡️ Create hydrophobic layer, repels water and salt.
  • 🌟 Attached shine (unlike mastic).
  • 🔄 Easy to update (once every 6-12 months).

How to apply:

  1. Wash and degrease the thresholds.
  2. Apply wax with a sponge or microfiber (thin layer).
  3. Polish after 10-15 minutes with a dry cloth.

Wax will not protect against mechanical damage (for example, from a brush at a car wash), but it will prevent chemical corrosion from reagents. For maximum effect, combine with liquid plastic (wax on top).

💡

Wax compounds are the only option that can be applied to paintwork without the risk of damaging the paint. All other methods require stripping down to metal or soil.

6. Professional formulations: when maximum protection is needed

If you are not ready to spend time on processing or want guaranteed results for 5+ years, use professional systems:

  • 🔝 Dinitrol 4941 — Swedish mastic based on synthetic resins (lasts up to 7 years).
  • 🔝 Tectyl ML — semi-liquid composition for hidden cavities + external processing.
  • 🔝 Noxudol 700/1000 — two-component epoxy mastic (used at service stations).

These compounds:

  • 💪 Withstand salt water, gravel, UV radiation.
  • 🕒 Serve 5-10 years no update.
  • 🔧 Required special equipment (application gun, IR drying).

The cost of treatment at a service station: from 5,000 rubles (only thresholds) to 15,000 rubles (full anticorrosive). If you decide to do it yourself, buy Dinitrol in cans (about 2000 rubles/liter) and follow the manufacturer’s instructions.

7. Common mistakes: why processing doesn’t work

Even an expensive anticorrosive agent will last less than a year if you make these mistakes:

  • 🚫 Application on a damp surface → detachment after 1-2 months.
  • 🚫 Ignoring rust → corrosion will continue to spread under the protection layer.
  • 🚫 Too thick layer → cracks due to temperature changes.
  • 🚫 Lack of soil → mastic/plastic will not adhere to metal.
  • 🚫 Washing in the first 48 hours → washing off the uncured composition.

Another common problem is incompatibility of compositions. For example:

  • Bitumen mastic + liquid plastic = peeling.
  • Wax + zinc spray = chemical reaction (appearance of white spots).
  • Movil + mastic = bubbles and cracks.

If you are not sure about compatibility, do a test: apply the compositions to small area (for example, on the inside of a door) and check the reaction after 24 hours.

8. Frequently asked questions

Is it possible to treat the sills without removing the wheels?

Yes, but miss it properly internal cavities and upper part of the threshold it won't work. For complete processing you need:

  • Remove the wheels (to access the arches and the lower edge of the sill).
  • Remove the plastic covers (if any).
  • Use flexible nozzle for the can for hidden areas.

Without removing the wheels you will only protect visible part (about 40% of the surface).

How often should processing be updated?

The period depends on the composition and operating conditions:

Composition Service life (years) Signs of wear
Bitumen mastic 1-2 Cracks, peeling, getting dirty
Liquid plastic 3-5 Scuffs, loss of shine
Zinc spray 2-4 Rust at joints, chips
Wax 0.5-1 Stops repelling water
Dinitrol/Tectyl 5-10 Mechanical damage

In cities with aggressive reagents (Moscow, St. Petersburg, Yekaterinburg) service life is reduced by 30-50%.

Is it possible to paint over anticorrosion?

Yes, but not all compositions are compatible with paint:

  • ✅ Can be painted:
    • Zinc sprays (after complete drying).
    • Liquid plastic (special paints, for example, Plasti Dip Paint).
    • Epoxy mastics (after priming).
  • ❌ Can't paint:
    • Bitumen mastic (the paint will peel off).
    • Wax compounds (the paint will not adhere).

For painting use aerosol acrylic enamels (for example, Motip or Kudo) and apply in 2-3 layers with interlayer drying for 15 minutes.

How to treat the thresholds from the inside?

The internal cavities of the thresholds protect liquid anticorrosives oil based:

  • 🛢️ Movil (classic, but flows when heated).
  • 🛢️ Tectyl ML (semi-liquid, does not flow).
  • 🛢️ Dinitrol 409 (for hidden cavities).

Technology:

  1. Remove the drain plugs.
  2. Rinse cavities anti-corrosion cleaner (for example, App Cleaner).
  3. Spray the composition through the holes (use flexible tube nozzle).
  4. Close the holes rubber plugs or aluminum tape.

⚠️ Attention: Do not use for internal processing bitumen mastic or liquid plastic - they clog drainage holes, which leads to moisture accumulation inside.

Is it worth treating the sills on a new car?

Yes, but not before 3-6 months of operation. Reasons:

  • Factory galvanized coating by itself protects the metal for the first 1-2 years.
  • Processing too early may breach the warranty (if the dealer detects third-party compounds).
  • In the first months of LCP sits down - anticorrosive may crack.

The best option for a new car:

  1. First 6 months: wash with waxy shampoo (for example, Liqui Moly Carnauba-Wash).
  2. After 6-12 months: treatment liquid plastic or zinc spray in vulnerable areas (lower edge of the threshold, joints).