Bondo, a brand name synonymous with auto body filler (popularized by 3M), is a staple in car repair for both professionals and DIY enthusiasts. This thick polyester resin compound is incredibly effective for quickly and seamlessly repairing dents, scratches, and minor damage on vehicles. When applied correctly, Bondo can restore your car’s body to its original smooth finish, saving you time and money. However, improper application can lead to uneven surfaces and costly rework.
This guide will walk you through the process of using Bondo auto body repair correctly, ensuring a professional-quality finish the first time. Learn how to prepare your vehicle, apply Bondo, and achieve a flawless repair that lasts.
Understanding Auto Body Filler (Bondo)
Auto body filler, often referred to as Bondo due to brand recognition, is essentially a polyester resin putty. It comes with a cream hardener that, when mixed, creates a workable, putty-like substance. This mixture is designed to fill imperfections and restore a vehicle’s body panels to a smooth, paintable surface after minor collisions or damage.
Think of Bondo as similar to spackle used in home repairs. Both are used to fill holes and even out surfaces before painting. The key difference lies in Bondo’s formulation, specifically engineered to bond with metal and withstand the harsh conditions a car endures.
It’s important to know that there are different grades of auto body filler available, each suited for varying degrees of damage and desired finish quality. Choosing the right type is crucial for a successful repair.
Types of Bondo and Body Fillers
You’ll generally find body fillers categorized into standard, medium, and premium grades. The severity of the damage you’re addressing will dictate the most appropriate type. While having all three on hand can be useful for tackling different projects, selecting the right one from the start is key to achieving optimal results and avoiding rework.
For those looking for specific brands, some of the most trusted and widely used auto body filler brands include:
- Bondo (3M): The most recognized name, offering a range of fillers for various needs.
- Dynatron: Known for its quality and ease of use, popular among professionals.
- Evercoat: Offers a wide variety of fillers, including premium options like Rage Gold, favored for smooth finishes.
- U-Pol: A global brand providing innovative body repair solutions.
- POR-15: Specializes in rust preventative and repair products, including specialized fillers.
- USC (US Chemical & Plastics): Offers a comprehensive line of auto body repair materials.
Choosing a reputable brand ensures you’re working with a product formulated for automotive applications and designed for durability and a quality finish.
Where and Where Not to Use Bondo
Bondo has unfortunately gained a somewhat negative reputation in some circles, often due to misuse rather than product flaws. The common mistakes stem from improper application techniques – applying it too thickly, skipping sanding steps, or using it in unsuitable locations. The truth is, body filler is a standard material in auto body repair, and most repaired vehicles will have some amount of filler.
The ease of application of Bondo can be a double-edged sword. Its user-friendly nature sometimes leads to overzealous application or usage in inappropriate situations.
Here are situations where you should NOT use body filler:
- Over Rust: Applying Bondo directly over rust is a major mistake. It will only conceal the rust temporarily and accelerate corrosion underneath, leading to bigger problems down the road.
- Full Gas Tanks: Bondo is flammable in its uncured state and, while gas-resistant once cured, should never be used directly on or to repair fuel tanks.
- High Heat Areas: Avoid using body filler in areas exposed to extreme heat, such as exhaust systems or engine components. The heat can cause the filler to degrade, crack, or bubble, resulting in a “popcorn ceiling” like texture and failure.
If you are dealing with rust, it’s crucial to properly remove all corrosion by sanding, media blasting, or even cutting out the rusted section and patching with new metal or mesh before applying any body filler.
So, where should you use body filler? Bondo is ideal for repairing:
- Hail Damage: Small dents caused by hail are perfect candidates for body filler.
- Minor to Medium Dents: Dents that aren’t too deep and don’t significantly distort the metal can be effectively filled.
- Deep Scratches: Scratches that penetrate below the paint level and into the metal require filler to restore a smooth surface.
- Small Holes: Minor holes, such as those from removed emblems or trim, can be filled with Bondo.
While small patch jobs might seem straightforward, they can be tricky to execute flawlessly. It’s generally recommended that individuals with some experience in auto body repair or skilled DIYers tackle these types of repairs. Learning to use Bondo effectively is achievable for beginners, but some initial practice and patience are beneficial.
Applying body filler using a spreader
Step-by-Step Guide: How to Use Bondo Auto Body Filler
Like any auto body project, proper preparation is key. Before you start applying Bondo, gather all the necessary tools and materials. For repairing dents and scratches with body filler, you will need:
- Body Shapers (Optional): For shaping metal before filling.
- Bondo Spreaders: For mixing and applying filler smoothly.
- Paintless Dent Repair Kit (Optional): For pushing out dents to minimize filler needed.
- Stud Gun (Optional): For pulling out larger dents.
- Dent Removal Tools (Optional): Various tools for manipulating metal.
- Body Filler (Bondo): Choose the appropriate type for your repair.
- Sanding Block: For even sanding.
- 80-grit Abrasive Sheet: For initial shaping and removing excess filler.
- 180-grit Abrasive Sheet: For feathering edges and refining the surface.
- Patch Metal or Mesh (Optional): For larger holes or rust repair.
- Mixing Board or Palette: A clean, non-porous surface for mixing.
- Cleaning Supplies: Soap, water, wax and grease remover.
- Gloves and Respirator: For personal safety.
At a minimum, you absolutely need the Bondo filler, hardener, spreader, and sandpaper. The rest of the tools are helpful and can improve the quality and efficiency of the repair, especially for more complex damage. Once you have everything ready, it’s time to prepare the surface of your vehicle and begin the repair process.
Step 1: Surface Preparation is Crucial
The first and most critical step is to thoroughly prepare the area you’ll be repairing. The vehicle’s surface must be completely clean and free from any contaminants that could interfere with Bondo adhesion. This includes bugs, grime, dirt, wax, grease, and any other residues. Start with a thorough wash using soap and water, followed by a wipe-down with a wax and grease remover.
Next, address any existing damage. If there are dents, try to pull or push them out as much as possible using dent repair tools. Remove any loose rust with a wire brush or sandpaper. Scratches should be cleaned and lightly sanded to remove any loose paint or debris.
The paint surrounding the damaged area also needs proper preparation. Bondo doesn’t adhere well to glossy paint, or wet/humid surfaces. Sand down the paint around the repair area using 80-grit sandpaper to create a “tooth” for the filler to grip. Extend the sanding a few inches beyond the damaged area to allow for proper feathering of the filler later.
Tip: Use painter’s tape to carefully mask off the area around the damage. This will protect the surrounding paint from accidental sanding scratches and keep the repair area defined.
Step 2: Mixing and Applying Bondo
Now it’s time to mix the Bondo body filler. Follow the mixing instructions provided with your specific Bondo product, as ratios can vary slightly. Generally, you’ll need to mix a small amount of cream hardener with the Bondo paste. The correct ratio is essential for proper curing. Too much hardener can cause the Bondo to cure too quickly, while too little might prevent it from hardening completely.
Dispense the desired amount of Bondo onto your mixing board. Then, squeeze out the correct amount of hardener – often described as a “golf ball” of filler to a “pea” of hardener, but always refer to the product instructions. Thoroughly mix the Bondo and hardener together using your spreader. Continue mixing until the color is uniform throughout, usually a consistent pink or light red, indicating the hardener is fully incorporated. Work quickly once mixed, as Bondo has a limited working time before it begins to harden.
Using your spreader, apply a thin, firm coat of Bondo to the damaged area. Press firmly to ensure good adhesion to the prepared surface. Avoid applying too much filler in the first coat. Focus on filling the dent or scratch, and slightly overlapping onto the surrounding sanded paint. It’s better to apply multiple thin coats than one thick coat, as thin coats cure more evenly and are less prone to cracking.
If you need to use body shaping tools to mold the Bondo to a specific contour, do so while the filler is still pliable but beginning to set up (often referred to as the “cheese” stage).
Allow the Bondo to dry completely. Drying time varies depending on the product, hardener ratio, temperature, and humidity, but typically takes around 20-30 minutes. The Bondo will become hard and no longer tacky to the touch when fully cured.
Step 3: Sanding and Shaping the Bondo
Once the Bondo is completely dry and hard, it’s time to sand it down to achieve the desired shape and smooth finish. Start with an 80-grit abrasive sheet wrapped around a sanding block. The sanding block ensures even pressure and prevents gouging. Begin sanding the high spots and excess filler to bring it down to the level of the surrounding body panel.
Use overlapping strokes and consistent pressure. Periodically check your progress by running your hand over the area to feel for high or low spots. If you sand too much and create a low spot, don’t worry. You can simply apply another thin layer of Bondo to fill the low area and repeat the sanding process.
As you get closer to the final shape, switch to 180-grit sandpaper to start feathering the edges of the Bondo into the surrounding paint. Feathering creates a seamless transition, making the repair invisible after painting. Continue sanding until the Bondo is smooth, level with the surrounding panel, and the edges are blended smoothly.
Step 4: Finishing – Primer, Paint, and Clear Coat
With the sanding complete and the Bondo perfectly shaped and feathered, the repair is ready for finishing. First, apply several coats of automotive primer to the sanded Bondo area and the feathered edges. Primer seals the Bondo, provides a uniform surface for paint adhesion, and helps to reveal any remaining imperfections that might need further attention. Lightly sand the primer with fine-grit sandpaper (e.g., 320-grit or higher) after it dries to ensure a perfectly smooth base for paint.
Next, apply your vehicle’s color-matched base coat paint, following the paint manufacturer’s instructions for application and drying times. Apply several thin coats for best coverage and color match.
Finally, apply a few coats of clear coat over the painted area. Clear coat protects the base coat paint and provides a glossy, durable finish that matches the original factory paint. After the clear coat is fully cured, you can further refine the finish by wet sanding with very fine-grit sandpaper (e.g., 2000-grit and higher) and polishing compound to achieve a show-quality shine.
Pro Tips for Using Bondo
To ensure your Bondo repair is successful, durable, and looks professional, keep these helpful tips in mind:
- Spreader Choice: While plastic spreaders are included in many Bondo kits and are disposable, metal spreaders are more durable, easier to clean, and can create sharper, more defined edges when applying filler.
- Mixing Ratios are Key: Always adhere to the correct hardener-to-filler ratio specified by the Bondo manufacturer. Incorrect ratios can lead to curing problems and a weak repair.
- Avoid Cardboard Mixing: Don’t use cardboard as a mixing palette. Cardboard is absorbent and can draw resins and hardeners out of the Bondo mixture, altering its chemistry and potentially weakening the repair. Use a plastic or metal mixing board.
- Have Thinner on Hand: In hot weather or if you’re working in a warm environment, Bondo can cure very quickly. Having a body filler thinner on hand can help extend the working time if needed. Use it sparingly and according to the thinner’s instructions.
- Use Heat Lamps in Cold/Wet Weather: If you’re working in cold or humid conditions, using heat lamps can significantly speed up the curing process of Bondo and ensure proper hardening. Be careful not to overheat the Bondo, and follow safety guidelines when using heat lamps.
Next Steps and Achieving a Flawless Finish
By following these step-by-step instructions and incorporating the pro tips, you can effectively use Bondo auto body filler to repair minor damage and restore your vehicle’s appearance. With practice and attention to detail, you can achieve results that are just as good as, if not better than, professional body shop repairs.
Ready to get started on your auto body repair project? Auto Body Toolmart offers a wide selection of body fillers, spreaders, dent repair tools, primers, and all the other supplies you’ll need to achieve professional-quality results. Browse our catalog and equip yourself with the best tools for the job!
