Replacing thresholds is one of the most labor-intensive operations when restoring a body, which inevitably violates the factory protection of the metal. After welding, zones with a high thermal load are formed inside the threshold structure, where the factory zinc burns out and the metal becomes susceptible to corrosion. If you ignore the quality anti-corrosion treatment immediately after welding, the new threshold can rot from the inside within 2–3 years, nullifying all repair efforts.
An ideal environment for the development of rust is created inside the closed profile: condensation, moisture from snow and reagents from the road accumulate there, and air access is difficult. The main task of the master is not just to paint over the seams, but to create a continuous, elastic film that will block the access of oxygen and moisture to the metal. In this article we will analyze a step-by-step algorithm of actions that will allow you to do anticorrosive professional level in a garage environment.
The quality of future protection directly depends on surface preparation and the choice of the right materials. Errors at the welding or initial cleaning stage can negate even the most expensive coating. Therefore, it is important to understand the physics of the processes: when welding, the metal heats up to 1500 degrees, losing its protective properties, and requires immediate isolation from the aggressive external environment.
Preparing the internal cavity after welding
The first and most critical step is to thoroughly clean the welds and surrounding areas. After installing a new threshold, they remain inside slag, scale and possible rust residues that must be removed mechanically. Use a flap sander or special brushes on your drill to get to clean metal in all accessible places.
⚠️ Attention: Before starting stripping, be sure to remove all flammable materials from the interior and trunk. Sparks from sanding can burn through the floor sheathing or cause a fire.
Pay special attention to the junction of the threshold with the side member and body pillars. This is where microcracks and crevices most often form where water flows. If you find through holes or severe metal defects, they need to be welded or soldered before applying any protective compounds. The surface should be dry, grease-free and matte for better adhesion.
For degreasing, use special alcohol-based compounds or anti-silicone. It is better not to use gasoline or thinner 646, as they can leave a greasy film or damage the exterior paintwork if not handled carefully. After degreasing, allow the surface to dry for 10–15 minutes.
☑️ Preparing the threshold cavity
Selecting materials to protect hidden cavities
The anticorrosion market offers many solutions, but not all are suitable for treating thresholds from the inside. The main requirement for the material is the ability to penetrate microcracks and not harden over time. Solid bitumen mastics in closed cavities quickly crack from vibration and temperature changes, ceasing to perform their function.
The optimal choice is oil anticorrosion agents or wax-based formulations with corrosion inhibitors. Oil compositions have excellent penetrating ability and displace moisture, remaining liquid for years. Wax coatings create a stronger but more flexible film that is also resistant to mechanical damage.
For primary protection of welds, acidic primers (phosphate) are often used, which chemically bond to the metal, preventing oxidation. However, they can only be applied to cleaned areas, and the top must be covered with a basic anticorrosive agent. Primer is not a finishing coating.
| Material type | Base | Service life | Features of application |
|---|---|---|---|
| Oil anticorrosive | Non-drying oils | 3–5 years | Displaces moisture, penetrates cracks, requires regular updating |
| Wax anticorrosive | Paraffin/Wax + solvent | 5–7 years | Creates an elastic film, resistant to stone impacts |
| Acidic soil | Phosphoric acid | Up to 1 year (as a base) | Only under the coating, cannot be left unprotected, it will etch the metal. |
| Cannon fat | Petroleum products | 1–2 years | Cheap, but quickly washed off and loses properties at high temperatures |
Anticorrosion spray application technology
For high-quality processing of the internal cavities of thresholds, a special gun with a long nozzle (spear) is required, which allows the composition to be supplied into the depth of the profile through technological holes. Regular spraying with a brush or sponge will not give the desired result, since you will not physically reach the far corners of the structure.
The application process begins by installing the spray gun at the deepest point of the cavity. After turning on the material supply, slowly pull the gun towards you, evenly covering the walls. It is important not to rush: the layer should be greasy, but not dripping. The compressor pressure is usually regulated within 4–6 atmospheres, depending on the viscosity of the composition.
The nuances of working with different temperatures
When working in a cold garage (below +15°C), oil compounds may become too viscous. It is recommended to heat the container or jar with anticorrosive agent in warm water to 30–40 degrees before use. This will improve the sprayability and penetration of the material.
Pay special attention to the weld areas. Go over them additionally, making sure that the composition gets into every pore and microcrack. If the threshold has a complex internal structure with reinforcements, try to change the angle of insertion of the nozzle in order to process hidden pockets.
Treatment of welds and hard-to-reach places
The weld seam is the most vulnerable place where the metal was exposed to maximum thermal effects. The structure of the metal here is changed, and corrosion begins from here. Before applying the main layer of anticorrosion, the seams can be treated with a rust converter (if there are pockets) or an acid primer to create a chemical barrier.
After the soil has dried (usually 30–60 minutes), the main layer of protection is applied. In hard-to-reach places where the gun nozzle does not fit, you can use the wicking method. To do this, the anticorrosive agent is applied with a brush to the inlet hole, and due to the illary action (capillary effect), it flows into the narrow gaps between the metal and the amplifiers.
Don't forget about the drainage holes at the bottom of the sills. They must remain open so that condensation can escape freely. You can’t fill them “to capacity” - this will create a bathtub effect, and the water will constantly come into contact with the metal, accelerating decay.
⚠️ Attention: Never seal or seal drainage holes after treatment. Lack of ventilation and drainage will lead to rapid accumulation of moisture inside.
Restoration of sound insulation and assembly
After the anticorrosive agent has dried (the polymerization time depends on the composition and ranges from 12 to 24 hours), you can begin to restore the sound insulation. For thresholds, bitumen-polymer vibration insulators or felt mats are usually used. It is important that the material is moisture resistant, otherwise it will absorb condensation and become a source of corrosion.
Before applying the vibration insulation sticker, degrease the outer surface of the threshold. Roll the material with a roller, removing air bubbles. If you use felt, make sure it does not block drainage holes or fit too tightly into areas where water contact may occur.
Use masking tape to temporarily secure the sound insulation before final rolling. This will allow you to try on the material and cut it exactly along the contour without damaging the fresh anticorrosive material.
The final stage is the installation of plastic covers and plugs. Check their tightness. If water gets into the interior through the cracks, it can ruin the carpet and cause an unpleasant damp smell. If necessary, replace old clips with new ones.
Common mistakes and quality control
One of the most common mistakes is applying anticorrosive to a damp or poorly cleaned surface. Water sealed under a layer of oil or wax will continue to destroy the metal, and the rotting process will be hidden from view. Always check the humidity in the room and the dryness of the metal.
Another mistake is saving material. “You can’t see it with your eyes, so it’s not necessary.” The internal cavities of the thresholds must be covered evenly. An insufficient layer will quickly dry out or be washed off, leaving the metal unprotected. It’s better to waste an extra can of compound than to boil the thresholds again in a year.
Carry out quality control visually through the technological holes using a flashlight. The coating should be uniform, without bald spots or streaks. If you find any defects, they must be corrected before assembling the vehicle.
The quality of anti-corrosion treatment depends 80% on surface preparation and only 20% on the brand of the selected material.
How often should threshold processing be repeated?
The recommended interval for inspection and renewal of anticorrosive coatings is 2–3 years for oil-based compositions and up to 5 years for high-quality wax coatings. When operating in harsh conditions (sea salt, reagents), it is better to reduce the interval to 1.5–2 years.
Is it possible to apply anticorrosive paint over old paint?
Yes, it is possible and necessary. Anticorrosive protects not only bare metal, but also prevents paint damage due to corrosion. The main thing is that the old paint holds tightly and does not have blisters.
Do I need to remove the thresholds for high-quality processing?
Removing the thresholds (if these are removable elements) allows for troubleshooting and cleaning to be carried out as efficiently as possible. However, for most modern cars, the thresholds are a permanent part of the body, and processing is carried out through technological holes.
Is anticorrosive material harmful to rubber seals?
Most modern compounds are inert to rubber and plastic. However, aggressive solvents (such as those found in some cheap converters) can cause the rubber to swell. Always read the material manufacturer's instructions.