Through corrosion of the metal in the lower part of the doorway requires immediate intervention, since ignoring the problem leads to rapid destruction of the power structure of the body. A car owner who discovers a through hole must understand that simply painting or covering it with putty will not stop the rotting process, but will only temporarily hide the defect. The choice of specific material for restoration depends on the depth of damage, available equipment and the planned budget for renovation.
There are several proven methods for fixing holes, each of which has its own technical features and applications. The most reliable method is considered to be welding a new metal insert, however, for a temporary solution or in the absence of a welding machine, composite materials based on epoxy resins can be used. It is important to correctly assess the degree of destruction metal before starting work, so as not to waste resources on restoring an element that is easier to replace entirely.
The initial assessment of the threshold condition determines the further strategy of action and the choice of consumables. If rust has covered more than 60% of the element's area, it makes sense to consider a complete replacement of the part, since local repairs may not be economically feasible. In situations where the damage is localized in the form of individual spots or small holes, it is possible to use restoration technologies without dismantling the entire assembly.
The key success factor is high-quality surface preparation, which includes removing all loose oxides and degreasing the work area. The use of unsuitable compounds on a poorly prepared substrate will lead to peeling of the material and the resumption of corrosion within a short period of time. May require angle grinder, rust converter and special primers to ensure adhesion.
Corrosion assessment and surface preparation
Before choosing how to repair a hole, you must thoroughly clean the damaged area down to bare metal, removing all blisters of paint and loose rust. The use of mechanical cleaning means, such as wire brushes or sandpaper, allows you to identify the real boundaries of the damage, which are often wider than visible to the eye. Only after complete removal of oxides can a decision be made on the applicability of a particular method recovery.
If, after stripping, the metal is found to be thinned to the point of foil or has multiple holes through it, a more serious approach than simply applying sealant will be required. In such cases, cutting out the damaged segment and installing a patch is often used.
⚠️ Attention: When cleaning thresholds with an angle grinder or drill, be sure to use a respirator and safety glasses, as metal dust and rust particles can enter the respiratory tract and eyes, causing serious injury.
Degreasing the prepared surface is a critical step that cannot be ignored. Residues of oils, silicone or dirt prevent normal adhesion of repair compounds to the car body. For this purpose, it is best to use specialized anti-silicone solvents that do not leave a greasy film after evaporation.
- 🛠️ Mechanical cleaning to bare metal, removing all pockets of corrosion.
- 🧪 Treatment with a rust converter to neutralize remaining oxides in hard-to-reach places.
- 🧼 Thorough degreasing of the work area before applying any materials.
- 🔍 Visual inspection of the back side of the threshold (through technological holes) to assess the scale of the disaster.
Welding as the most reliable restoration method
Semi-automated gas shielded welding (MIG/MAG) is considered the gold standard for repairing through-corroded sills, providing maximum joint strength and durability. The essence of the method is to cut out the damaged area and weld in a new metal insert made from a sheet of similar thickness. This approach restores not only tightness, but also structural integrity body
Performing high-quality welding requires some skill, since it is easy to burn through the thin metal of the thresholds, creating new holes instead of eliminating old ones. Experienced craftsmen use interrupted seam or pulse welding techniques to minimize thermal deformation of the part. After cooling, the seams are cleaned and treated with anti-corrosion compounds to prevent the reappearance of rust.
☑️ Welder Checklist
If you don't have access to professional equipment, you can consider resistance welding or using special soldering materials for body work, although these are less reliable. It is important to understand that any welding operation requires subsequent high-quality anti-corrosion treatment the internal cavity of the threshold, where moisture often gets in. Without protection from the inside, even the highest quality external repairs will not save the part from rapid destruction.
Welding safety
When carrying out welding work near fuel lines and plastic elements, it is necessary to use asbestos or special heat-resistant screens to protect against burning and fire. Welding work should be carried out in a well-ventilated area or outdoors.
Use of epoxy compounds and fiberglass
For medium-sized holes where welding is not possible or practical, two-part glass fiber reinforced epoxy resins are excellent. This method is often called “cold welding”, although the physical and chemical process is completely different. Fiberglass acts as reinforcement, absorbing mechanical loads, and epoxy adhesive ensures tightness and adhesion to metal.
The application technology involves layer-by-layer laying of the material: first a layer of resin is applied, then fiberglass is applied, which is again impregnated with resin. The number of layers depends on the size of the hole and the required strength; Thresholds typically require 3-5 layers to create a hard shell. After polymerization, such a “sandwich” becomes very hard and resistant to vibrations and temperature changes.
It is important to follow the mixing proportions of the components specified by the manufacturer, since a violation of the technology will lead to the composition not hardening or having low strength. Before applying epoxy, the surface must be rough for better adhesion, so it is recommended to lightly sand smooth metal with coarse sandpaper. The finished surface can be sanded, puttyed and painted after complete drying.
Application of polyurethane foam and aluminum mesh
The combined method of using polyurethane foam and aluminum mesh (or fiberglass mesh) is a popular budget solution for sealing large holes before finishing. The foam serves as a volume filler and heat insulator, and the mesh, recessed into a layer of foam or putty, creates a load-bearing base. This method does not restore the strength structure, but effectively seals the interior from dust and water.
The process is as follows: the hole is covered with a temporary backing, a small amount of foam is carefully blown inside, which, after expansion, is cut flush with the body. A mesh is placed on top of the cured foam using epoxy glue or putty, creating a durable membrane. This method is good because it makes it easy to repeat complex geometric shapes of thresholds.
⚠️ Caution: Polyurethane foam is hygroscopic and absorbs moisture, so it should not be left open or in contact with air inside the cavity. Be sure to ensure that the outer layer is completely sealed, otherwise the metal under the foam will rot even faster.
After forming a base of foam and mesh, the surface is leveled with automotive putty. Such a “pie” is sufficiently rigid to withstand washing and use, but is inferior to metal in impact strength. However, for older cars where aesthetics are more important than factory durability, this is a viable option recovery appearance.
Comparison of materials for repairing thresholds
The choice of material directly affects the durability of the repair and the labor required to complete it. Each method has its advantages and disadvantages, which should be taken into account when planning work. Below is a comparative table of the main methods for eliminating holes in thresholds.
| Method | Strength | Difficulty | Durability | Cost |
|---|---|---|---|---|
| Metal welding | High | High | 10+ years | Average |
| Epoxy + fiberglass | Average | Average | 3-5 years | Low |
| Foam + mesh | Low | Low | 1-2 years | Very low |
| Aluminum patches with glue | Average | Low | 2-4 years | Low |
As can be seen from the table, welding is the uncontested leader in strength and service life, but requires skills and equipment. Composite methods can achieve good visual results at a lower cost, but require more frequent condition monitoring. For a temporary solution or selling a car, less reliable options may be suitable.
Finishing and anti-corrosion protection
Regardless of the chosen method of sealing the hole, the final stage should always be high-quality anti-corrosion treatment. The repair area and especially the internal cavities of the threshold must be protected from moisture, salt and reagents. Oil or wax compounds are ideal for internal cavities, as they displace water and create an elastic film.
After repair, the outer part is puttied, primed and painted in body color or protected with an anti-gravel coating. Anti-gravel creates a rough layer that perfectly resists the impacts of small stones and sand flying from under the wheels. Neglecting this stage will lead to the fact that in a year or two the repair will have to be redone.
- 🎨 Using acidic primer for primary protection of metal before puttying.
- 🛢️ Use of Movil or analogues for processing internal hidden cavities through technological holes.
- 🪨 Applying anti-gravel composition to the lower part of the threshold for mechanical protection.
- ☀️ Drying and polymerization of all applied layers in accordance with the manufacturer’s instructions.
Regular inspection of thresholds after the winter period will allow you to notice in time the beginning pockets of corrosion and eliminate them before new holes form. Proper care and timely car washing with underbody treatment significantly extends the life of the body. Remember that water and dirt are the main enemies of metal, and the tightness of protective layers is critical.
⚠️ Attention: Do not seal the technological holes in the thresholds tightly after treatment; air access is necessary to evaporate condensate if it does form inside. Use special valves or mesh plugs.
FAQ: Frequently asked questions
Is it possible to fill a hole in the threshold with putty only?
No, regular automotive putty is not designed to fill through holes. It does not have sufficient adhesion to air and does not have reinforcing properties. The putty will quickly crack and fall off. For holes, you definitely need a base: metal, fiberglass or mesh.
How long does it take to dry epoxy resin with fiberglass?
The polymerization time depends on the air temperature and the type of hardener used. Typically, initial setting occurs in 30-60 minutes, but the composition gains full strength after 24 hours at a temperature of +20°C. During the cold season, the process may take longer.
Is it necessary to completely remove the thresholds for a quality repair?
Preferable, but not always required. Complete removal allows for high-quality welding and processing of the insides from all sides. Local repairs are also possible on a car, but the quality of anti-corrosion protection of internal cavities in this case will be lower.
What is the best way to paint a repaired threshold?
It is best to use acrylic or polyurethane enamels in combination with an anti-gravel coating on the bottom. It is important to match the exact paint code of your vehicle so that the repair stain does not stand out.
Will a rust converter help if the hole is already through?
A rust converter neutralizes oxides, but it cannot restore missing metal. Its use makes sense only at the stages of preparing the surface around the hole, but it will not be able to seal the hole itself.