Repairing a car threshold: how to properly fill a hole and avoid mistakes

Why puttying a threshold is not just “covering up a hole”

The thresholds of a car are one of the most vulnerable areas of the body. They are constantly attacked by dirt, salt, sand and moisture, which leads to corrosion, chips and even through holes. Many drivers mistakenly think that it is enough to simply “cover up” the damage with putty - but this approach is guaranteed to lead to re-rusting within a few months. The point is that high-quality threshold repair requires an integrated approach: removal of rust to “living” metal, proper processing of edges, use of specialized materials and adherence to application technology.

In this article we will look at step by step instructions for filling holes in the threshold - from preparing tools to final painting. You will learn which materials to choose for long-lasting results, how to avoid common mistakes (for example, using regular putty instead of fiberglass for through holes), and what to do if corrosion has managed to “eat” the metal over a large area. The instructions are suitable for both beginners and those who have already had experience in body repair, but want to deepen their knowledge.

We will pay special attention three critical stages, which are often ignored: proper removal of rust (spoiler: sandpaper is not enough!), choice of putty depending on the size of the hole and final anti-corrosion treatment. If you skip at least one of these steps, the repair will last at most a year - or even less.

Tools and materials: what you need for work

Before you begin the repair, prepare everything you need. The final result depends 50% on the quality of materials and tools. Here minimum set, which you can’t do without:

• 🔧 Grinder or drill with brush attachment - to remove rust and old paint. Alternative: Hand scraper and sandpaper P80-P120, but it will take 3 times longer.
• 🧴 Rust converter (for example, Tsinkar or Kudo KV-70010) - for processing metal after stripping.
• 🧰 Fiberglass putty (for through holes) + finishing putty (for alignment). Popular brands: Novol, Body, 3M.
• 🖌️ Spatulas - metal (for fiberglass) and rubber (for finishing putty). Optimal sizes: 5–10 cm and 15–20 cm.
• 🔥 Primer for metal (preferably two-component) and anti-gravel (for protection against chipping).
• 🎨 Spray paint (to match the body color) + varnish. For an accurate selection, use the color code of your car (usually indicated on the plate under the hood).
• 🧤 Protective equipment: respirator (dust), gloves, goggles. Putty and primer are toxic!

If the hole in the threshold is more than 5 cm in diameter, additional fiberglass or aluminum cover (sold in auto stores). It serves as a “reinforcing” layer and prevents the putty from falling through. For large areas of corrosion (more than 10 cm) it is better to use welding repair - putty will be a temporary solution here.

⚠️ Attention: Do not use to repair thresholds regular universal putty (for example, for walls or furniture). It cannot withstand vibrations, moisture and temperature changes - after 2-3 months it will fall off in pieces. Only automotive materials!

Preparing the threshold: removing rust and preparing the surface

This is the most important and time-consuming stage. If you leave even microscopic pockets of rust, they will continue to spread under the putty, and after a year you will have to redo the job. follow the algorithm:

1. Washing and degreasing. Wash the threshold thoroughly with car shampoo, then wipe white spirit or antisilicon (for example, APP W700). This will remove grease, wax and other contaminants that will interfere with the adhesion of the materials.
2. Removing rust. Using a grinder with a flap wheel or a drill with a wire brush, clean the damaged area until pure metal. If the rust is deep, use cup attachment for drill. Main rule: clean 2–3 cm more than the visible corrosion area — rust often “disappears” under the paint.
3. Processing by converter. Apply product (eg Tsinkar) with a brush for 10–15 minutes, then rinse with water and dry. The converter stops corrosion and creates a protective layer.
4. Cleaning up the edges of the hole. If the hole is through, cut the edges with a grinder at an angle of 45° - this will improve the adhesion of the putty to the metal.

For visual inspection use flashlight: Point the light at an angle - this way you will even see small spots of rust that you might have missed. If, after stripping, the metal is too thin or full of holes (for example, rusted through), you will have to cut out the damaged area and weld the patch. Putty won't help here!

Remove dirt and grease|Strip off rust to bare metal|Treat with converter|Trim the edges of the hole at an angle|Dry the surface-->

Choosing putty: which one is suitable for holes in the threshold

Not all putties are the same. Used to repair thresholds three types of materials, each of which solves its own problem:

Type of putty	What holes is it suitable for?	Benefits	Disadvantages
Fiberglass (for example, Novol Fiber)	Through holes up to 5 cm, deep dents	High strength, does not fail, withstands vibrations	Difficult to sand, requires skill when applying
With aluminum dust (for example, Body 989)	Holes 3–7 cm, areas with thin metal	Good adhesion, resistance to temperature changes	More expensive than fiberglass, takes longer to dry
Finish (for example, Novol Finish)	Leveling the surface before painting	Easy to sand, perfectly smooth surface	Not suitable for filling holes - only for the final layer

For holes of size more than 5 cm putty alone is not enough - you will need reinforcing mesh or fiberglass cover. It is glued with epoxy glue (for example, Poxipol) on the back side of the threshold, and then putty is applied. If the hole is very large (from 10 cm), it is better not to risk it and contact straightener — he will cut out the rusted section and weld a new one.

⚠️ Attention: Never mix putty with solvent or acetone to speed up drying! This disrupts the structure of the material and it will lose strength. If you need to speed up the process, use infrared lamp (keep at a distance of 30–40 cm).

Step-by-step instructions: how to apply putty to the threshold

Now we move on to the most important stage. Follow the algorithm and take your time - each layer must dry. Here step by step plan:

1. Preparation of putty. Mix the main component with the hardener (the proportions are indicated on the package, usually 100:2). Stir only with a spatula - Do not shake to prevent air bubbles from forming. The finished mass should be homogeneous, without lumps.
2. Applying the first layer. For through holes use fiberglass putty. Apply it with reverse side of the threshold (if there is access), forming a “substrate”. Then fill the hole from the front side, squeezing out the putty with a putty knife so that it penetrates deep. For convenience, you can use plastic spatula — it is flexible and follows the shape of the threshold.
3. Reinforcement (if necessary). If the hole is larger than 3 cm, after the first layer, glue fiberglass mesh (sold in rolls) and apply a second coat of putty over it. This will prevent cracks.
4. Alignment. After drying (usually 15–30 minutes at +20°C), clean the surface sandpaper P80, then apply finishing putty thin layer for perfect smoothness.
5. Sanding. Use sandpaper P120→P240→P400, gradually reducing the grain size. Grind criss-crossto avoid waves. To control the smoothness, run your hand over the surface - it should be perfectly smooth.

The thickness of the putty layer should not exceed 3–4 mm. If the hole is deeper, apply putty in several layers, allowing each to dry. Otherwise, the material may crack. To speed up drying (for example, at low temperatures), use construction hair dryer, but keep it at a distance of 20–30 cm so as not to overheat the putty.

Primer and painting: how to protect the threshold from new corrosion

Putty is only half the battle. Without proper priming and painting, the threshold will begin to rust within a few months. Here step by step process final processing:

• 🧽 Degreasing. Wipe the sanded surface antisilicon (for example, APP W700). Do not use regular solvent - it leaves a film.
• 🎨 Primer. Apply 2-3 coats two-component primer (for example, Novol Protect 360) with intermediate drying for 10–15 minutes. The primer protects the putty from moisture and improves paint adhesion. For thresholds it is better to use primer with anti-corrosion additives.
• 🛡️ Anti-gravel treatment. Apply to the lower part of the threshold (which is in contact with the road surface). anti-gravel (for example, Body 930). This will protect against chipping from stones and sand. Apply 2 layers with drying time 1 hour.
• 🖌️ Painting. Use car enamel in a can (select by body color code). Apply in thin layers at a distance of 20–25 cm, holding the can at a 45° angle. Last layer - clear varnish (for example, Novol Clear) for added protection.

After painting, let the threshold dry minimum 24 hours before use. In the first 3-5 days, avoid high-pressure washing and off-road driving - paint and varnish only gain final strength after a week.

⚠️ Attention: If you are painting the threshold in winter or in a garage without heating, use quick drying materials (for example, soil Novol Rapid) and infrared lamp for drying. At temperatures below +10°C, ordinary paint may not polymerize, and the repair will go down the drain.

Common mistakes and how to avoid them

Even experienced craftsmen sometimes make mistakes when puttingtying thresholds. Here TOP-5 misseswhich lead to rapid re-rusting or peeling of the material:

• 🚫 Saving on materials. Using cheap putty (eg "Made in China" without title) or low quality primers. Consequences: cracks, peeling, corrosion after 3–6 months.
• 🚫 Incomplete removal of rust. If you leave even a small fire, it will “grow” through the putty. Check the metal needle or awl - if the tool fails, the rust remains.
• 🚫 Thick layer of putty. The optimal thickness is up to 3–4 mm. If more, the material will crumble. For deep holes use fiberglass overlays.
• 🚫 Painting without primer. Paint applied directly to putty will peel off quickly. Soil is a mandatory intermediate layer!
• 🚫 Ignoring anti-gravel treatment. The rapids are constantly attacked by rocks and sand. Without anti-gravel, the paint will flake off over the winter.

Another common mistake is quick repair no drying between coats. If you apply paint to wet putty, it will become cloudy and bubble. Always comply interlayer drying time (indicated on the packaging of materials).

What to do if the putty cracks after drying?

If the cracks are small (up to 1 mm), sand them with sandpaper P120, degrease and apply a thin layer of finishing putty. If the cracks are deep, remove the entire layer and redo the work with fiberglass reinforcement. The cause is usually too thick a layer or improper mixing with the hardener.

FAQ: Answers to frequently asked questions

Is it possible to putty the threshold without welding if the hole is 10 cm?

Technically possible, but it will be temporary solution (for 1–2 years). For such large holes, it is optimal to cut out the rusted area and weld a patch. If welding is not possible, use aluminum or fiberglass cover, sealed with epoxy glue, and then putty. However, remember: under heavy loads (for example, hitting a curb), such repairs may not withstand.

Which putty is better to fill holes in thresholds? VAZ 2107 or Niva?

Ideal for domestic cars with their problematic sills fiberglass putty with aluminum dust (for example, Body 989). It is resistant to vibrations and temperature changes, which is important for machines without anti-corrosion treatment. For VAZ 2107 and Niva It is also recommended to apply after puttying anti-gravel in 2 layers — these cars are often used off-road.

How long does it take for putty to dry on a threshold at a temperature of +15°C?

At +15°C the drying time increases by approximately 1.5 times compared to +20°C. For example:

• Fiberglass putty: 40–50 minutes instead of 25–30.
• Finishing putty: 30–40 minutes instead of 15–20.

To speed up the process, use infrared lamp (keep at a distance of 30 cm) or construction hair dryer (warm air mode, not hot!).

Is it possible to paint the threshold with a brush and not with a spray can?

It is possible, but the result will be worse. The brush leaves streaks and bubbles, but the spray can gives an even finish. If you still paint with a brush:

• Use soft natural bristle brush (for example, maklovitsa).
• Thin the paint thinner (10–15%) for better spreading.
• Apply thin layers crosswise, allowing each to dry for 20–30 minutes.

After painting with a brush, be sure to polish the threshold GOI paste or abrasive polish to remove traces of stubble.

How to treat the threshold from the inside after puttying?

The inside of the threshold must be protected no less carefully than the outside. Use:

• Anti-corrosion wax (for example, Tectyl or Dinitrol) - applied with a brush or spray.
• Movil - a budget option, but requires re-processing once a year.
• Epoxy primer (for example, Body 960) - for additional metal protection.

Dry the threshold thoroughly before processing construction hairdryer - Moisture inside will accelerate corrosion.