The thresholds of a car are one of the most vulnerable areas of the body, which is the first to take the blows of road chemicals, moisture and mechanical damage. This is where pockets of corrosion most often arise, which can turn metal into a sieve in a couple of years. If you notice blistering paint or red streaks, you need to act immediately, otherwise repairs will require replacing entire sections.
Many car owners mistakenly believe that surface treatment with “anti-rust” will solve the problem, but this is only a temporary disguise. High-quality restoration requires complete removal of rust, competent welding (if necessary) and adherence to the technology of applying layers of protection. In this article we will look at how to do these jobs professionally using available tools.
Before taking active action, it is necessary to assess the scale of the disaster. Sometimes the external manifestation is just the tip of the iceberg, and a through hole is hidden under the decorative trim. Diagnostics metal condition is the first and most important stage on which the choice of repair method depends.
To work, you will need not only paint, but also a serious set of tools: an angle grinder, a welding machine (if welding is planned), a grinder and personal protective equipment. Don't neglect respirator and safety glasses, as dust from rust and old paint is toxic and harmful to the lungs.
⚠️ Attention: Never strip metal in a closed garage without strong ventilation. Dust from epoxy primers and corrosion products can cause severe allergic reactions and poisoning.
Diagnosis and damage assessment
Before purchasing materials, you should determine the depth of corrosion. Light surface deposits and deep through holes require fundamentally different approaches. Use a sharp object or magnet to check the integrity of the metal underneath the blisters. If the tool fails, it means the metal is completely destroyed and requires replacement.
Rust is often hidden under plastic trim or in places where moldings are attached. Dismantling all elements are required, otherwise you will miss hidden sources that will continue to destroy the body from the inside. Visual inspection should be supplemented by tapping: a dull sound may indicate peeling of metal or the presence of a large amount of putty.
If the damage affects less than 15-20% of the threshold area, local repairs can be done without overcooking. However, if there are multiple through holes or destruction of the internal structure of the spar, the only correct solution will be threshold replacement entirely. Saving at this stage will lead to the fact that in six months corrosion will appear again.
Required tools and materials
The quality of the result directly depends on the materials used. Cheap primers and paints may not provide adequate adhesion and protection, especially under aggressive winter conditions. The basis of durability is epoxy primer, which creates an impenetrable film that blocks oxygen from reaching the metal.
To clean the surface you will need an abrasive tool. Flap wheels on a grinder or grinding attachments on a drill will help you quickly remove rust to bare metal. It is important not to overheat the metal when grinding, so as not to disturb its structure and not to deform thin body elements.
The list of necessary consumables should include:
- 🛠️ Degreaser (anti-silicone) - to remove oils before painting.
- 🧴 Epoxy primer is the base layer of corrosion protection.
- 🎨 Acrylic filler primer - for smoothing out small irregularities.
- 🖌️ Automotive enamel and varnish - finishing decorative coating.
- 🧽 Fiberglass putty - for filling deep dents and joints.
Pay special attention to the choice putties. For thresholds, it is critical to use compounds that are resistant to vibration and moisture. Regular polyester putty can crack over time, so for areas with high loads, elastic options or special compositions with aluminum powder are preferable.
Use degreaser in two stages: before priming and immediately before painting. Residues of silicone from hands or polishes can cause defects in the finishing layer (“craters”).
Surface preparation and cleaning
The most labor-intensive stage is the complete cleaning of the damaged area. All rust, old paint and primer must be removed until clean, shiny metal appears. The islands of corrosion left behind will continue to grow under the new coat of paint, nullifying all efforts.
Use a grinder with a grinding wheel for rough sanding and a flap wheel for finishing sanding. The boundaries of the transition from the cleaned area to the whole paint should be smooth, without sharp steps. This will ensure good adhesion of the materials and the absence of visible boundaries after painting.
If plastic trims were installed on the threshold, check the condition of the metal underneath them. Often this is where moisture and dirt accumulate, causing hidden corrosion. Rinse thoroughly and dry all hard-to-reach areas using compressor for purging hidden cavities.
⚠️ Attention: When working with an angle grinder, beware of sparks. Seal the windows, headlights and rubber seals around the work area with masking tape and cardboard to prevent them from being burned or scratched.
After mechanical cleaning, the surface must be treated with a rust converter (if micro-foci remain) and thoroughly degreased. Clean metal should be free of dust, fingerprints or oil stains.
Welding work and anti-corrosion treatment
If corrosion has eaten through the metal, welding of a repair insert or a new threshold will be required. For this purpose, semi-automatic gas shielded welding (MIG/MAG) is used. It is important to follow the technology: weld with short seams, allowing the metal to cool to avoid warping of the thin body metal.
Weld seams must be cleaned with a grinder flush with the main plane. After this, a special welding primer is applied to them to prevent oxidation of the seam. This step cannot be skipped, since the weld is the first to rust.
The internal cavities of the thresholds require separate processing. It is blown inside through the technological holes anti-corrosion composition (for example, based on wax or oils), which creates a protective film on the inner wall. This prevents the metal from rotting from the inside, where paint does not get in.
Why can’t thresholds be welded using conventional electric welding?
Conventional arc welding produces too high a temperature, which leads to burnout of thin metal (0.6-0.8 mm) and severe thermal stress. This causes deformation of the body geometry and the appearance of cracks in the heat-affected zone.
To protect welds and cut points, you can also use zinc-containing primers, which provide cathodic protection. Zinc sacrifices itself, taking the corrosion on itself while the iron remains intact. This is especially true for the lower parts of the body.
Puttying and leveling the plane
After priming and sanding, small irregularities may remain, which can be removed by puttying. A layer of putty with fiberglass is applied to the prepared surface if it is necessary to fill deep holes or metal joints. For finishing leveling, light finishing putty is used.
It is important to follow the mixing proportions of putty and hardener specified by the manufacturer. A lack of hardener will result in the material not drying, and an excess will make it too brittle. The components must be mixed thoroughly until a uniform color without streaks is obtained.
The putty is sanded dry or wet (depending on the type of material) using a block. This allows you to maintain a flat surface and avoid making holes. You can control the evenness of the surface by touch or by applying a ruler.
The table below will help you navigate the types of putties and their purposes:
| Type of putty | Purpose | Features |
|---|---|---|
| With fiberglass | Sealing through holes and deep dents | High strength, difficult to sand, requires overlapping |
| Universal | Leveling out medium unevenness | Good adhesion, medium drying time |
| Finish | Removing minor scratches and marks | Easy to sand, gives a smooth surface |
| Aluminum | Areas with high temperature or vibration | Heat resistance, elasticity |
☑️ Putty quality control
Priming and painting thresholds
The final stage is the application of paint and varnish. The first layer is epoxy primer, which insulates the metal. After it has dried and lightly sanded, an acrylic filler primer is applied, which hides the sanding marks and creates a base for the paint.
The paint is applied in 2-3 layers with interlayer drying, following the manufacturer's recommendations for exposure time. It is important to set up the spray gun correctly: the torch should be even, without spitting. Thresholds are often painted “in transition” to hide the difference in color with the rest of the body, especially if the car is not new.
After the paint has dried (usually 20-30 minutes after the last layer), varnish is applied. It adds depth to the color and protects the enamel from ultraviolet radiation and mechanical stress. The varnish is also applied in 2 layers.
To protect against stones and gravel that fly from under the wheels, it is recommended to additionally treat the lower part of the threshold anti-gravel coating. This is a rubber-like composition that absorbs the impact of small particles, preventing them from penetrating the paint layer.
⚠️ Attention: Painting should be carried out in a clean, well-lit room at a temperature not lower than +18°C. Dust settling on fresh paint will ruin the entire appearance, and low temperatures will prevent the varnish from drying properly.
The quality of painting depends 80% on surface preparation. Thorough sanding and degreasing is more important than the brand of paint used.
Frequently asked questions (FAQ)
Is it possible to paint the sills without removing the plastic trim?
Highly not recommended. Rust is almost always hidden under the linings. If you paint only the visible part, corrosion will continue under the plastic, and in a short time the threshold will rot completely. In addition, it is almost impossible to properly disguise the paint transition boundary under the overlay.
Do the sills need to be removed from the car for repairs?
In most cases this is not required. Modern technologies make it possible to weld and paint thresholds on site with high quality. Removal is necessary only in case of total corrosion, when the entire element is changed and access to the internal reinforcements is required from all sides.
How long after painting can you wash your car?
Complete polymerization of the paint coating takes from 2 to 4 weeks. You can wash your car under pressure after 7-10 days, but you need to do this carefully, using a soft sponge and neutral shampoo. Aggressive chemicals and brushes are prohibited in the first month.
Which anticorrosive agent is best to use for thresholds?
For external surfaces that will be painted, bitumen mastics or liquid plastic are best suited to which paint adheres well. For internal cavities, use wax- or oil-based penetrating compounds (such as Moville, Dinitrol), which displace moisture and do not dry out.