Maya Ellison
Plastic body filler, commonly referred to by the brand name Bondo, is a lightweight polyester-based plastic body filler. It is used to fill small imperfections on vehicle body parts. When mixing plastic body filler, it is important to use a nonporous surface and a plastic or metal spreader. The filler should be mixed thoroughly in one direction, reducing air pockets, which can cause pinholes. The ratio of filler to hardener is crucial and will depend on the specific product being used. It is important to work quickly and efficiently as the filler can cure rapidly. Once the filler is mixed, it can be applied to the desired surface, ensuring that it is pushed into any crevices and that there is enough extra filler left above the surface level to sand the repair smooth.
| Characteristics | Values |
|---|---|
| Common names | Bondo, Polyester filler, Plastic filler, Mud |
| Use case | Filling small imperfections on vehicle body parts |
| Application | Use a plastic or metal spreader to apply the filler |
| Application technique | Work in small sections, gradually building up the filler to the desired thickness |
| Application technique | Feather the edges to blend the filler seamlessly with the surrounding surface |
| Application technique | Pay attention to contours and curves, adapting your application technique to different surface shapes |
| Application technique | Apply several thin coats rather than one thick coat |
| Mixing | Use a nonporous surface and a plastic or metal spreader to mix your filler |
| Mixing | Mix in one direction, by folding and spreading in one direction |
| Mixing | Mix thoroughly until the filler is one uniform color |
| Mixing ratio | 1:1 ratio of inch diameter of filler to inch-length of hardener |
| Curing | Let your plastic filler cure before priming |
| Surface preparation | Apply a high-quality automotive primer to the repaired area |
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What You'll Learn
Use a non-porous surface and spreader
When mixing plastic body filler, it is important to use a non-porous surface and spreader. This is because the filler needs to be mixed without trapping air bubbles, which can cause pinholes in your bodywork. Using a non-porous surface, such as plastic, steel, or any other surface that is not porous, ensures that the filler does not absorb any of the resins.
You can use a plastic or metal spreader to mix the filler, depending on your preference. Metal spreaders offer a more defined edge and tend to last longer, but plastic spreaders are cheaper and can be cut to match specific shapes. It is important to work quickly and efficiently when mixing the filler, as it can cure rapidly. Always follow the manufacturer's instructions for mixing, and ensure that the filler is thoroughly and evenly mixed to achieve the best results.
When applying the filler to the repair area, use a spreader or applicator and work in small sections, gradually building up the filler to the desired thickness. It is recommended to apply several thin coats rather than one thick coat. Push the filler into any crevices and wipe it as smooth and level as possible, leaving extra filler above the surface to sand it down.
Feather the edges of the filler to create a seamless transition between the repaired area and the original surface. This helps to blend the repair smoothly and achieve a professional finish. Remember to always apply the filler to a cleaned and prepped surface, covering a larger area than just the dent for feathering.
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Mix filler and hardener in a 1:1 ratio
When mixing plastic body filler, it is important to get the ratio of filler to hardener correct. If you add too much hardener, the mixture will gel too quickly. Conversely, adding too little will increase the curing time, and if you don't add enough, it will never cure. The recommended ratio is a 1:1 ratio of filler to hardener.
To begin, scoop out a golf ball-sized amount of filler and place it on a mixing board or flat surface. Flatten the filler into a circle. You can use a piece of cardboard, sheet metal, or glass glued to a wooden board. The advantage of glass is that it can be easily scraped clean with a razor after each use.
Next, add the hardener. Squeeze out about 5-8 drops of hardener or one line across the top of the filler circle. The amount of hardener you use will depend on how quickly you want the mixture to dry. One line of hardener will take about 15 minutes to dry, while two lines will take about 5 minutes.
Now, it's time to mix the two substances together. Use a paint stick or a plastic or metal spreader. Avoid whipping or circular motions, as these can introduce air bubbles. Instead, fold the substances together until they are fully combined and you see an even colour. The mixture should have a putty-like consistency and a pinkish or light bluish colour.
Once the filler and hardener are mixed, you can apply them to the repair area. Always follow the directions provided with your specific product for the best results.
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Mix in one direction to avoid air bubbles
When mixing plastic body filler, it is important to avoid whipping the mixture as this will incorporate air bubbles, which will result in pinholes in your bodywork. To avoid this, mix the filler in one direction using a slow and deliberate figure-X pattern, wiping the filler across itself. This will reduce air pockets, which become pinholes in your bodywork.
It is also important to mix the filler on a non-porous surface such as metal or plastic. Do not use cardboard as it will absorb critical chemicals and weaken the filler. Mix only as much filler as you can easily spread in about 10 minutes. This is usually about a 1/2-inch line of catalyst to a golf ball-sized gob of filler, but follow the mixing instructions on the product you purchased.
Before mixing, massage the tube of catalyst to mix it and warm it. Stir the filler thoroughly as it separates in storage. When mixing, use a spatula to scrape up the hardener and smush it into the filler. Smear it out, then scrape up the semi-mixed pile at a 90-degree angle from the smear and repeat.
Always mix the filler and hardener to the exact proportions stated on the packaging. Adding too much hardener will cause an excess amount of pinholes due to the heat generated by the catalyst.
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Apply filler to a larger area than the dent
When applying filler to repair a dent, it's important to work on an area larger than the dent itself. This is known as feathering and will help to blend the repair smoothly into the surrounding surface.
Firstly, prepare the surface by removing at least three inches beyond the dent. You'll need to sand the area to ensure the filler will stick. For this step, you can use heavier sandpaper, such as 150-grit. It's crucial to remove enough material so that you are applying the filler to bare metal.
Next, you'll need to mix the body filler. Body filler is a two-part product, consisting of a base filler and a hardener. The exact ratio of filler to hardener may vary, so be sure to follow the instructions on your chosen product. Mix the two parts together on a non-porous surface, such as plastic or steel, using a paint stick or a plastic spreader. Mix in one direction, spreading and folding, until the filler is one uniform colour.
Now you can apply the filler to the prepared area, using a plastic spreader. Push the filler into any crevices, ensuring it is smooth and level, but leave extra filler above the surface level so that you can sand it down to a smooth finish. It's better to apply several thin coats rather than one thick coat, so don't be afraid to apply a second or third coat if needed.
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Sand and finish with regular body filler
To sand and finish with regular body filler, you must first ensure that the area to be filled is thoroughly dry and free from wax, grease or dust. Use a powered grinder or sander to remove any existing paint and primer down to the metal surface, creating a rough surface for the filler to adhere to. For larger repairs, it is best to strip the area down to bare metal.
Next, mix the body filler with the correct hardener, following the manufacturer's instructions. Apply the filler firmly to the repair, building it up until it is slightly higher than the surrounding panel. Use a steel rule to apply the filler, bending it to the shape of the panel to highlight any dents. Mark out the area to be filled with a pencil, then use a plastic scraper to apply the filler.
Once the filler has hardened (after about 15-20 minutes), you can begin sanding. Start with 40-60 grit production paper to sand the filler into shape. Be careful not to remove too much of the filler, especially if using rotary sanding disks. Switch to a flat sanding block with abrasive paper attached for the final shaping. Sand in straight lines, regularly changing direction to avoid grooves and low spots. Never sand in a circular motion.
To finish, switch to a finer grade paper, such as 180-280 grit. This will help to remove any remaining scratches and create a smooth surface for painting. You can also use a glaze or icing over the filler to fill in any remaining pinholes and scratches. This will create an even smoother surface and make it easier to sand.
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Frequently asked questions
Plastic body filler is a two-part product that requires mixing with an activator or hardener. It is important to follow the manufacturer's instructions and use the recommended ratio of filler to hardener. Use a clean, non-porous mixing surface and mix the filler in one direction, folding and spreading, until it is one uniform colour.
You can use a paint stick to mix the filler and knead the hardener. For application, use a plastic or metal spreader to apply the filler to the desired surface. Metal spreaders offer a more defined edge and longer lifespan, while plastic spreaders are cheaper and can be cut to match specific shapes.
The ideal ratio is about one golf-ball-sized amount of filler to a dime-sized amount of hardener. This can vary depending on the manufacturer's instructions, so always check the recommended ratio.
Avoid stirring or whipping the mixture, as this will cause air bubbles which will lead to pinholes. Instead, slowly pull the filler across itself in an X pattern to reduce air pockets.
You will have approximately 5 to 10 minutes to apply the filler before it starts to harden, so work quickly but efficiently.
