How to treat sills inside a car against corrosion

Rotten metal inside the threshold is detected when, when tapped, a characteristic dull sound is heard, and the paint on the outside is already swollen. The internal cavity of the element is subject to destruction faster than the external one due to the constant accumulation of condensate and the impossibility of proper ventilation. It is hidden corrosion that leads to a loss of geometric rigidity of the body, which is critical for safety in a side impact. Threshold processing inside requires full access to technological holes and the use of specialized compounds capable of penetrating microcracks.

The restoration process begins with the dismantling of the plastic linings, if they are designed by the manufacturer. On some models, access is achieved through holes in the underbody or by removing the seats and carpeting. Ignoring the preparatory steps leads to the fact that an expensive anticorrosive agent is applied to wet or dirty metal, which reduces the effectiveness of the procedure to zero.

High-quality protection does not simply mean blowing it with “cannon fat”, but a set of measures to stop oxidative processes. Modern paraffin or oil-based compounds displace moisture and create an elastic film that does not crack when the body vibrates. It is important to understand that bitumen mastics are not suitable for hidden cavities, as they quickly dry out and crumble.

Diagnostics of the condition of hidden cavities

Before taking active action, it is necessary to objectively assess the scale of the disaster. Visual inspection of the external part often does not provide a complete picture, so the use of an endoscope or at least a flashlight with a flexible mirror is required. Through technological holes in the ends of the thresholds, the condition of the metal is checked: the presence of loose rust, through holes or remnants of factory anticorrosive.

If you hear a ringing sound when you tap, the metal is most likely intact. A dull sound indicates delamination of the metal or the presence of a thick layer of rust and dirt. In places of welded joints, pockets of corrosion often form, which quickly spread along the inner surface. Threshold diagnostics must be thorough so as not to miss hidden sources of destruction.

Particular attention should be paid to the areas around the door and arch fastenings. This is where moisture and reagents from the road most often get in. If through holes are found, simple processing will no longer help - the elements will need to be re-welded. A thickness gauge can be used to assess metal thickness, although it works better on external surfaces.

⚠️ Attention: If diagnostics reveals through holes or the metal crumbles from light pressure with a screwdriver, mechanical cleaning can lead to the final destruction of the element. In such cases, consultation with a bodybuilder is required.

The inspection results are recorded for planning the scope of work. It often turns out that one threshold requires only prevention, while the second requires serious intervention. Saving time at the diagnostic stage can lead to repeating the procedure a year later.

Tool preparation and materials selection

The success of the event directly depends on the quality of the chemicals used and the availability of special tools. The market offers many solutions, but not all are suitable for internal cavities. The main requirement is high penetrating power and the ability to displace water. Oil anticorrosives are considered most effective for hidden areas, as they remain elastic for decades.

• 🛢️ Anti-corrosion compounds: Choose oil-based products (eg Dinitrol, Noxudol, Rust Stop) or waxes with corrosion inhibitors.
• 🔧 Stripping Tool: brushes on a flexible cable, sandpaper on a long handle, rust converter.
• 💨 Application equipment: an air gun with a long nozzle (probe) or aerosol cans with a tube.
• 🧤 Consumables: rags, degreaser, masking tape, gloves, respirator.

For mechanical cleaning of rust inside a closed cavity, special flexible brushes are used. They allow you to peel off the loose layer of oxides without damaging healthy metal excessively. After mechanical cleaning, the surface must be treated with a chemical rust converter, which converts iron oxides into stable compounds.

The choice of anticorrosive agent depends on the operating conditions of the vehicle. For regions with harsh winters and large amounts of reagents, compositions with a high solid content are preferable, which create a denser armor. However, for internal cavities, it is fluidity that is important so that the composition gets into all corners. Do not use nitro enamels or paints for interior treatment - they do not have anti-corrosion properties and will crack quickly.

Comparison of anticorrosive types

Oily compounds penetrate into the smallest pores and displace water, but are washed off with aggressive chemicals when washing. Paraffin creates a more resistant film, but penetrates worse. Bituminous ones are absolutely not suitable for hidden cavities.

Cleaning and degreasing technology

Surface preparation is the most labor-intensive stage, on which 80% of success depends. Applying protection to dirt or rust is of no use. The process begins with the removal of all removable elements: plastic linings, rubber plugs, carpets in the cabin. Access should be maximum.

Mechanical cleaning is done carefully so as not to make through holes. If the rust is deep, a rust converter can be used. It is poured inside, kept according to the instructions, and then thoroughly washed with water under pressure to wash off the reaction products. After washing, the cavity should be completely dry.

Degreasing is a mandatory final step before applying anticorrosive. A special degreaser or white spirit is used. The surface must be free of oil, dirt or moisture. Degreasing thresholds ensures adhesion of the protective composition to the metal.

Drying the internal cavity is a critical moment. The moisture remaining inside, preserved by the anticorrosive agent, will continue to destroy the metal. To speed up the process, you can use a hair dryer (with caution) or a compressor with warm air. It is better to leave the car in a warm garage for a day before applying the topcoat.

The process of applying a protective composition

The application technology depends on the selected material. If an aerosol can is used, it must be warmed to room temperature (20-25°C) for better fluidity. The nozzle-tube is inserted into the technological hole until it stops, after which, pressing the trigger, the composition is sprayed, gradually pulling the tube towards itself.

It is important to cover the entire inner surface with an even layer. Movements should be smooth, covering the entire perimeter. For hard-to-reach areas such as welds and corners, the pressure in the tube helps the compound penetrate deeper. Anti-corrosion treatment must be carried out at a temperature not lower than +10°C.

Stage	Action	Features
1. Preparation	Cleaning and drying	Removing loose rust
2. Priming	Converter application	Only if there are pockets of corrosion
3. Degreasing	Solvent wipe	To a clean rag
4. Application	Anticorrosive spraying	Even layer all around
5. Final	Drying and assembly	Putting the plugs in place

After applying the base coat, it is recommended to let it set for 15-30 minutes and then apply a second coat to ensure coverage. Particular attention is paid to welding areas and metal joints, since this is where corrosion most often begins. Excess composition that gets onto visible parts of the body or into the interior must be immediately removed with a rag before it hardens.

Treatment of welds and hard-to-reach places

Welds inside thresholds are high-risk areas. When welding, the metal heats up, the zinc coating burns off, and the structure of the metal changes, making it more susceptible to rust. Therefore, seams require special attention when processing. The anticorrosive agent must penetrate to the very depths of the joint.

To process seams, special spray nozzles are used that create a fan jet at an angle of 360 degrees. This allows the entire inner circumference of the pipe or profile to be covered. If there is no access to the seam, the composition must be fluid enough to flow there by gravity and capillary effect.

⚠️ Attention: Do not try to seal the technological holes with sealant until the anticorrosive has completely dried. Air circulation must remain inside, otherwise condensation will be preserved.

In places where the threshold connects to the spar or arch, moisture often accumulates. These areas need to be shed additionally with the composition. Sometimes it is necessary to drill an additional hole (6-8 mm in diameter) in an inconspicuous place in order to properly process a hidden camera if the standard holes are not enough. Subsequently, this hole is closed with a rubber plug.

Errors during self-processing

Processing on your own often leads to errors that waste your efforts. One of the main ones is applying the composition to wet metal. The water remaining inside is sealed with anticorrosive and continues to destroy the body from the inside. Another common mistake is using inappropriate materials, such as used motor oil or grease, which quickly dry out or wash off.

Insufficient surface preparation is also fatal. If loose rust is not removed, it will continue to grow under the protective layer, tearing the anticorrosive material away from the metal in layers. In addition, many people forget to clean the outer part of the threshold from dirt before removing the linings, and it gets inside during dismantling.

The lack of protection for the interior and external elements is another nuance. Anticorrosives have a strong odor and can stain the upholstery, which is then impossible to wash. It is also important not to clog the drainage holes in the lower part of the thresholds, intended for water drainage, with the composition. Errors with anticorrosion may cost more than the procedure itself if the work has to be redone or the body repaired.

Ignoring safety precautions when working with chemicals can lead to poisoning or fire. Vapors from solvents and anticorrosives are explosive and toxic. Work should be carried out in a well-ventilated area or outdoors using a respirator.

Maintenance frequency and control

Even the highest quality anticorrosion agent does not last forever. Under the influence of vibrations, temperature changes and aggressive environments, it gradually loses its properties. Oily compounds can gradually flow down or be washed out, while paraffin ones can crack in severe frosts. Therefore condition monitoring thresholds must be regular.

The recommended frequency of inspection and renewal of protection is once every 2-3 years for new cars and annually for cars with mileage of more than 5-7 years. If the car is operated in harsh conditions (constant trips on primers, presence of reagents on the roads), the interval is reduced.

During scheduled maintenance, you can ask the technician to remove one of the plugs and assess the condition of the composition inside. If it is dry, cracked, or not there at all, it’s time to repeat the procedure. Timely updating of a thin layer of protection is cheaper than fighting serious corrosion.

In conclusion, it is worth noting that high-quality treatment of the sills inside is an investment in the longevity of the car. Preserving the strength structure of the body guarantees not only the absence of holes, but also the safety of passengers. The right approach to choosing materials and following technology will allow you to forget about rust for many years.

How to extend the life of anticorrosive

Avoid high pressure washing in the sill area. Check drain holes regularly for blockages. If chips appear on the outer part of the threshold, touch them up immediately.

Is it possible to use used motor oil to treat thresholds?

Strongly not recommended. The waste contains combustion products, acidic compounds and metal shavings, which can accelerate corrosion. In addition, it has an unpleasant odor and does not have the necessary anti-corrosion additives.

Do I need to remove the thresholds for high-quality processing?

In most cases, removing the sills is not necessary and even harmful, since dismantling can disrupt the geometry of the body or damage welded points. High-quality processing can be carried out through technological holes using long nozzles.

How often should the anticorrosive treatment be repeated?

The optimal interval is once every 2-3 years. In harsh climatic conditions or during active use on winter roads with reagents, it is recommended to carry out inspection and local updating annually.

What is the difference between the anticorrosive agent for hidden cavities and the composition for the bottom?

Compounds for hidden cavities are more liquid, flowable and have high penetrating abilities to flow into crevices. Bottom compounds are thicker, contain reinforcing fibers and are designed to protect against stone impacts and abrasive wear.