How to shed car sills with your own hands: a complete guide

Hidden corrosion is a silent body killer that can turn a once presentable car into a sieve in just a couple of seasons of active use. Particularly vulnerable in this regard rapids, which are closed cavities where moisture, dirt and reagents accumulate, causing irreversible destruction of the metal from the inside. Many owners notice the problem too late, when the paint on the outside is already swollen, but timely prevention allows you to avoid expensive repairs and loss of market value of the car.

Self-guided procedure anti-corrosion treatment requires a certain skill, the right tools and, most importantly, quality materials. You should not rely on cheap bitumen mastics, which over time crack and allow moisture to pass through, causing even more active rusting. In this article we will analyze a professional approach to the question of how to properly seal thresholds so that the protection lasts for many years.

High-quality work done with your own hands often surpasses the result obtained in dubious services, where craftsmen save time and materials. You don’t just have to “pour” something inside, but create a durable barrier that can withstand vibrations, temperature changes and the aggressive effects of road chemicals. Let's look at all the stages in detail.

Causes of hidden corrosion of thresholds

Understanding the nature of rust helps to combat it more effectively. Thresholds are complex structural elements consisting of outer and inner parts welded together. During operation, water inevitably enters these cavities through drainage holes or microcracks in the welds. When the car cools down, moisture condenses, mixing with road dust, salt and reagents, forming an aggressive electrolyte.

The situation is aggravated by the fact that the factory anti-corrosion coating inside the thresholds it is often applied fragmentarily or completely absent at welding points. Mechanical damage, such as impacts from stones or gravel, compromises the integrity of the outer protection, allowing oxygen direct access to the metal. If you ignore small chips, the oxidation process spreads deeper, covering hidden cavities that cannot be reached with a regular brush.

⚠️ Attention: Winter vehicle operation significantly accelerates corrosion processes due to the use of chloride reagents on roads, which act as rust catalysts even at subzero temperatures.

A particular danger is posed by the so-called “capillary corrosion”, when moisture penetrates into the gaps between the metal layers and the sealant. In a confined space without access to air, ideal conditions are created for the development of rust spots, which are not visually visible until the metal burns through. This is why regular inspection and updating of protection is critical.

Owners of cars with existing signs of corrosion should understand that simply painting the outside will not solve the problem. It is necessary to stop the process from the inside, neutralize existing lesions and create a new protective film. Only an integrated approach guarantees the safety of body elements.

Selection of an effective anti-corrosion composition

The market offers many metal protection products, but not all of them are suitable for treating hidden cavities. For thresholds the ideal choice is oil anticorrosion agents and special wax-based compositions that have high penetrating ability and do not dry out completely. Unlike bitumen mastics, such materials remain elastic, filling all microcracks and not cracking when the body vibrates.

One of the most popular and time-tested solutions is the use of drugs based on motor oil with the addition of special additives or finished products like Movil, NGM-ML or their modern analogues. These compounds displace moisture, envelop the metal with a thin film and block the access of oxygen. It is important to choose products that will not wash off with water, but will not destroy the factory rubber seals.

For the initial stage, when rust has already appeared, but there are no through holes yet, it is necessary to use rust converters. They chemically convert iron oxides into stable compounds, creating a protective layer. However, you cannot rely on them alone - the main preservative must be applied on top. The wrong choice of product can result in the protection being washed off after the first serious pressure wash.

Necessary tools and workplace preparation

High-quality processing is impossible without proper tools. The basic element is air gun for anticorrosion with a flexible hose and a special nozzle nozzle (often called a “turret”), which sprays the composition 360 degrees. Using conventional garden sprayers or brushes will not give the desired result, since the stream will not reach the remote corners of the threshold.

You will also need a compressor with a capacity of at least 200-300 liters per minute to ensure stable pressure. Indispensable for preparatory work drill with a brush or sandblaster if the corrosion is severe. Don't forget to prepare a solvent (for example, White spirit or antisilicone) for degreasing surfaces before applying protection.

The workplace must be well ventilated, as vapors from solvents and anticorrosives are toxic. It is ideal to carry out work in a garage with an air temperature of at least +15°C, since the cold composition is poorly sprayed and loses its properties. Be sure to use personal protective equipment: respirator, goggles and gloves.

It is important to provide access to internal cavities in advance. To do this, you often have to drill technological holes in the thresholds or remove plastic covers, if any. The diameter of the drill is usually 10-12 mm, which allows you to freely insert the spray tube. The drilling sites are subsequently sealed with special plugs.

Surface preparation technology for processing

Preparation is 80% of the success of the entire operation. If you apply anticorrosive agent to dirt or loose rust, it will simply peel off along with them, and the rotting process will continue. First of all, the car must be thoroughly washed, paying special attention to the arches and lower part of the body. After washing, the body should dry completely, ideally in a warm box.

This is followed by mechanical cleaning of problem areas. Using a drill with an attachment or sandblasting, remove the swollen paint and loose layer of rust to bare metal. Rust converter applied to oxide residues that cannot be removed mechanically. Once the converter has dried, the surface becomes black and hard, ready for the base coat.

⚠️ Attention: Do not apply anticorrosive agent over a wet rust converter - this will lead to fogging and lack of adhesion; the protection will peel off in layers in a month.

The final stage of preparation is degreasing. The surface is wiped with a rag soaked in solvent to remove dust, chemical residues and grease stains. Only a perfectly clean and dry surface will receive a high-quality layer of protection. Neglecting this stage negates all efforts.

To access the internal cavities, it is necessary to dismantle the plastic moldings if they block access to the technological openings. Some car models require removal of the wheels and fender liners for better visibility and access to the tube entry points.

The process of applying anti-corrosion protection

The pouring process itself requires care and adherence to technology. The gun tube is inserted into the technological hole, after which the composition is supplied under pressure. The tube should be moved slowly, starting from the far edge of the threshold (closer to the center of the car) and gradually moving towards the outlet, evenly turning the nozzle.

It is important not to overdo it with pressure, so as not to damage the factory seams or tear off the internal sound insulation, if any. The composition should settle into a thin but uniform film. For hard-to-reach places where the jet cannot reach, you can use a long brush or brush dipped in anticorrosive to manually coat the joints and welds.

After treating all cavities, excess composition that gets on visible parts of the body, glass or rubber seals must be immediately removed with a rag containing a solvent. It will be extremely difficult to wash off frozen anticorrosive. Pay special attention to doorways and sealing areas.

The technological holes through which the injection was carried out are closed with rubber plugs. If there are no standard plugs, you can make them yourself from thick rubber or use special plastic caps. The holes can be additionally coated with sealant for reliability.

Quality control and reprocessing times

After completion of the work, the car must be given time for final polymerization of the composition. This usually takes 24 to 48 hours depending on the ambient temperature and the type of material used. During this period, it is better to refrain from driving on dusty roads or washing under high pressure.

Quality control can be carried out visually by looking into the technological holes with a flashlight, or by selectively drilling a small control hole in an inconspicuous place to make sure that there is an anti-corrosion film on the inner wall. A high-quality applied composition should not have gaps or bubbles.

The service life of the protection depends on many factors: the quality of preparation, the selected material and operating conditions. On average, oil compositions require updating once every 1-2 years, as they are gradually washed out. Wax and more durable compositions can last up to 3-4 years, but annual inspection is required.

If you notice that after winter black slurry is dripping from under your car, this is a good sign - it means that the anticorrosive agent is working, washing away accumulated dirt and salt, and continues to protect the metal. If rusty water flows, the corrosion process has not been stopped, and the procedure must be repeated.