How To Use Auto Body Filler: A Complete Guide

Dents and scratches are a reality for car owners, but fixing them doesn’t have to be complicated or expensive. Auto body filler is a simple yet powerful solution for repairing these issues; you can use it to rebuild and smooth out the surface, making it look like the problem was never there. The trick is to use it the right way: a messy application can yield uneven results that send you back to square one.

Fortunately, auto body filler is relatively easy to use. In this blog, the team at Red Label Abrasives will guide you step-by-step through every part of the process, from cleaning the surface and applying filler to shaping it, and getting a paint-ready finish. By the end, you’ll have the confidence to repair your vehicle with results you’ll be proud to show off.

What is Auto Body Filler?

Auto body filler, sometimes called "Bondo," is your go-to solution for fixing dents, scratches, and other surface damage on your car. This putty-like material mixes a polyester resin with a hardener to create a durable surface you can sand, prime, and paint. It’s engineered to fill gaps and level out uneven areas, giving you a smooth finish that looks just like new. 

Tools and Materials You’ll Need  

To get the best results from your auto body repair, make sure you have all the necessary tools and materials ready before starting. Here’s a complete list of what you’ll need:  

• Auto Body Filler and Hardener: These are the core materials for filling dents and imperfections. Be sure to buy a high-quality brand for best results.  
• Mixing Board and Spreader: These tools allow you to apply and blend the filler and hardener.  
• Sanding Tools: Use sanding blocks, orbital sanders, or belt sanders to shape and smooth the filler as it cures.  
• Abrasives in Various Grits: Use coarse abrasives, such as 80-grit, to shape the filler, and finer grits, like 320-grit, to achieve a smooth surface for priming.  
• Cleaning Supplies: Degreasers, rags, and tack cloths can remove dirt, grease, and dust, making it easier for the filler to stick properly.  
• Primer and Automotive Paint: These finishing products bond to the filler and provide a professional-looking surface.  

Optional tools like body hammers and dollies can be useful for repairing larger dents. You can also use spot putty to cover smaller imperfections.

Preparing the Surface  

Preparation is the most important step in any auto body repair; if the surface isn’t ready, even the best filler won’t stick properly, and you’ll end up dealing with cracks and peeling. Here’s how to get best results.

• Clean the Surface Thoroughly: Start by cleaning the area completely. Use a degreaser to remove dirt, grease, wax, or any contaminants, as even a small amount of residue can prevent the filler from bonding securely. Use a lint-free cloth or a tack cloth to wipe everything down after degreasing.  
• Sand Down the Damaged Area: Once the surface is clean, it’s time to sand. Use coarse abrasives like 80-grit sandpaper or sanding discs to remove old paint, rust, or any loose material. Your goal is to expose clean, bare metal or fiberglass, depending on the material of your vehicle. If you’re working with stubborn rust, consider using a wire brush attachment or rust remover to make sure it’s completely gone.  
• Check for Additional Damage: Take a close look at the area once it’s sanded. Are there cracks, holes, or deeper issues that the filler can’t address? If so, you might need to weld the panel, use fiberglass repair materials, or even replace the panel entirely. Filler works best on shallow damage, so don’t try to use it as a fix-all. Addressing these deeper problems upfront will save you time and frustration later.  

Pro Tip: After sanding, the area will be covered in fine dust. Use a vacuum or compressed air to clear it away, then wipe the surface with a tack cloth to pick up any remaining particles. Dust can weaken adhesion, so take the extra time to get the surface spotless before you move on.    

Mixing the Filler Correctly  

Getting the mixture right is one of the most important steps in using auto body filler. A perfectly mixed filler is easy to apply, cures evenly, and gives you a lasting repair.  

• Determine the Right Ratio: Start by reading the manufacturer’s instructions, as every filler has a specific ratio of hardener to filler. Most products require a small amount of hardener (typically a pea-sized portion) for every golf-ball-sized scoop of filler. Use the mixing board to measure the amounts accurately. 
• Mix Thoroughly Without Trapping Air: Place the filler onto a clean mixing board, add the hardener on top, and start blending immediately. The goal is to avoid streaks, which indicate incomplete mixing. Be sure to fold the mixture rather than stirring it like you would a liquid. Stirring introduces air bubbles, which create pinholes and weaken the repair.  
• Work Quickly and Efficiently: Filler begins to harden in just a few minutes, depending on the product and the ambient temperature. This window of time is called the "pot life." Only mix as much as you can apply during that time. If you’re working on a large area, mix in smaller batches so that you’re not rushing and compromising the repair. Keep an eye on the consistency: if it starts thickening, it’s too late to use.  

Applying the Filler  

Applying auto body filler is where you bring the repair to life. In order to get a smooth surface, you'll need a steady hand and a methodical approach.

• Spread Thin Layers for Better Results: Always work with thin, even layers of filler. Use a plastic spreader to scoop the mixed filler and press it firmly onto the surface. This supports good contact and eliminates air gaps that can weaken the repair. Thick layers might seem like a time-saver, but they take longer to cure and are more likely to crack.  
• Build Up in Layers for Deep Repairs: For larger dents or deeper damage, don’t try to fix everything in one pass. Instead, apply the filler in multiple thin layers, letting each layer cure completely before adding the next. This method prevents sagging, enables better adhesion, and gives you more control over the repair. 
• Pay Attention to the Edges: As you spread the filler, feather the edges so they blend smoothly with the surrounding surface. This reduces the amount of sanding needed later and prevents hard lines that can show through the paint. Smooth as you go to save yourself extra work down the road.  

Pro Tip: Filler starts to cure quickly, and a dirty spreader can leave lumps or streaks in your application. Keep a rag or paper towel handy to clean your tools between layers for a flawless finish.  

Shaping and Sanding the Filler  

Once the filler has cured completely, it’s time to shape and smooth it to match the contours of your vehicle. This stage is where the repair starts to take its final form, and using the right abrasives is key to getting the results you want. 

Start with Coarse Grit for Shaping 

Begin with 80-grit sandpaper to remove excess filler and shape the repair. This coarse grit allows you to cut through the material quickly and establish the basic contour of the surface. Use a sanding block or an orbital sander to ensure even pressure and avoid creating dips or waves. 

Refine the Shape with Medium Grit 

Once you’ve shaped the filler, switch to 180-grit sandpaper to refine the surface and smooth out any rough areas. Sanding in a crisscross pattern supports consistency and helps prevent low spots or uneven transitions. 

Finish with Fine Grit

The final sanding step uses 320-grit sandpaper to create a surface that’s smooth enough for primer. Take your time when removing any minor scratches or imperfections. If you notice pinholes or tiny air pockets in the filler, apply a thin layer of spot putty, let it cure, and sand it again with 320-grit paper. 

Pro Tip:   Aluminum oxide abrasives are excellent for aggressive material removal while silicon carbide abrasives excel at fine finishing. Look for products designed specifically for automotive use (like our automotive sanding discs) to get the best results.  

Priming and Painting Over the Filler  

These steps protect the repair and restore the vehicle’s appearance. Attention to detail is extremely important here: skipping steps or rushing can leave you with a repair that’s noticeable or vulnerable to damage. 

Apply Automotive Primer

Start by choosing a primer that’s compatible with both the filler and your vehicle. Primers designed for automotive use bond to metal and filler, creating a strong, even foundation for paint.  

Spray the primer in thin, even coats, keeping the nozzle about 8–12 inches away from the surface to prevent drips or uneven coverage. Begin with a light tack coat to improve adhesion, then apply two to three full coats, letting each layer dry completely before applying the next. Drying times vary by product, so check the instructions for guidance. 

Sand the Primer  

Once the primer has dried, it’s time to refine the surface. Lightly sand it with fine-grit abrasives, such as 600-grit sandpaper, to eliminate any imperfections, like bumps or overspray. This step creates a smooth base for the paint to adhere to and prevents visible texture from showing through the topcoat.  

Use a sanding block for flat areas to maintain even pressure, and switch to hand sanding for curves and edges. If the primer feels rough or uneven, repeat the sanding process. Wipe the surface with a tack cloth afterward to remove dust.  

Apply Automotive Paint

The painting stage is where your repair starts to blend with the rest of the vehicle, so be sure to use high-quality automotive paint that matches your car’s color.  Hold the spray can or paint gun about 8–12 inches from the surface and apply the paint in thin, even coats. Start with a light tack coat to establish adhesion, then follow up with two or three full coats. Let each coat dry according to the manufacturer’s recommendations before applying the next.

Pro Tip: To blend the new paint with the surrounding area, feather the edges of the painted section into the existing paint. This technique supports a seamless transition by preventing harsh lines.  

Finish with a Clear Coat

The clear coat is your final layer This step seals the paint, protects it from environmental damage, and adds the glossy shine that makes your repair look like it came straight from the factory.  

Apply the clear coat in the same manner as the paint: thin, even coats with overlapping passes. Allow it to dry thoroughly between layers, and avoid touching the surface during this time. For extra protection and an even glossier finish, consider lightly wet-sanding the final clear coat with ultra-fine 1500- to 2000-grit sandpaper and polishing afterward.  

Add Premium Abrasives to Your Auto Repair Kit  

Using auto body filler is a skill that combines technique and patience. From surface preparation to priming and painting, each step contributes to a professional-quality repair. With practice, you can tackle even complex repairs with confidence.