How to Fix Dents with Bondo®
A rogue rock flung from a tire, an out of control shopping cart, your kid learning how not to park the hard way—there are endless ways to dent a car. And body work can get expensive fast. Many small dents today can be easily removed through “paintless dent removal,” in which dent removal specialists fix dents without the need to do paint or body work. That said, bigger, more complicated dents and creases in body panels require a bit more work.
In the old days, lead was used as a filler to fix and smooth creases and dents in the body panel repair process. Because this is a cumbersome and hazardous process, plastic 2-part fillers were invented to serve the same purpose. Plastic body fillers like Bondo soon became the norm. It’s important to note, Bondo is simply a brand of plastic body filler, and one of the first that’s been around for decades, so the term Bondo generally gets used as a generic term for all body filler. Body fillers have gotten a bad rep in some parts, but this is primarily due to poor use and prep. The truth is that pretty much any high-end custom car, restoration project, or collision repair where an entire panel could not be replaced has some body filler in it. That’s because when Bondo is used right and in the right applications and practice, it can fill low spots and become a permanent repair solution.
Follow this guide to learn how to fix dents and make it look like they were never there using Bondo, the fast and easy do-it-yourself solution. We’ll show you the sandpaper and tools needed to use body filler the right way.
How to Fix a Dent in a Car
Clean the Car and Remove Any Residue
Clean the vehicle with soap and water, followed by a residue remover. 3M Adhesive Remover is a great product for this step. This is important as any contaminants (wax, bug residue, tar, dirt) remaining when sanding begins are ground into the surface and could compromise the adhesion of repair materials.
Remove Paint from the Dented Area by Sanding
Use a sanding block or random orbital sander with an 80 grit abrasive sheet to remove all paint to approximately 2-5″ around the dent.
Start Working on the Dent with a Hammer, Mallet, or Dent Puller
When repairing any dent it is best to remove as much of the dent as possible prior to filling with body filler. An assortment of hammers and mallets, dollies, picks, slide hammers and dent pullers may be needed to perform this step. Always use appropriate eye and respiratory protection.
Once complete, keep in mind that plastic body fillers may adhere to bare metal, but it is not a good practice to do so, as metal can and will corrode and slowly release the bond from the plastic filler. This is why many repairs that are 5+ years old have plastic filler that has cracked. Once you feel your dent has been worked out, go back over your repair with sanding again and make sure the area is completely clean of any small rust pits, or other corrosion. Seal any bare metal with a good direct-to-metal primer and then provide a quick scuff of the primer with 220-300 grit sandpaper.
Mix Bondo and Put Down the First Layer of Body Filler
Using proper techniques mix Bondo® Body Filler using the Bondo® Plastic Spreader from 3M and apply mixed filler to the dent. The first layer should be applied with firm pressure to ensure good adhesion. Avoid over-application of filler as this just requires additional sanding. Work the filler out past the edges of the dent as you will need to shape and smooth the filler to blend into the surrounding panel. Let dry for 15 to 20 minutes, depending on temperature and humidity.
Shape the Filler with a Sanding Block
Select a sanding block that is appropriate for the size of the repair and attach an 80 grit abrasive sheet. Rough shape/sand the repair area. ALWAYS keep your block flat to the surface. Avoid over-sanding with 80 grit or the finished repair area will be low. If low spots are detected, it may be necessary to apply more body filler and repeat this sanding step.
Smooth Out the Filler with a 180 Grit Abrasive Sheet
Once satisfied with the rough shape of the repair, attach a 180 grit abrasive sheet to the sanding block and final shape the repair. After this step, the repair should be level and there should be no 80 grit scratches remaining. The body filler should also have a smooth featheredge all around the repair.
Use the Sanding Block to Remove Paint and Scratches
Attach a 180 grit abrasive sheet to the sanding block to featheredge the paint away from the body filler and remove all straight line scratches.
Mix and Apply Glazing and Spot Putty
Mix Bondo® Professional Glazing and Spot Putty and apply to the entire repair area overlapping the body filler and paint featheredge. Allow 15 minutes to dry, depending on the temperature and humidity.
Sand the Repair Area Until Level
Apply a 180 grit abrasive sheet to the sanding block and sand the repair area until level. Remember to keep the block flat to the surface at all times and do not oversand.
Remove Scratches from the Paint Around the Filler Area
Apply a 320 grit abrasive sheet to your sanding block and remove all straight line scratches from the paint around the filler area. At this point, you are ready to prime the area.
Prime with several coats as the initial coats will soak into the filler. Continue to sand with 220-320 as you prime over the area / sand several times. Check carefully for small pinholes, deep sand scratches, and continue to prime/sand or fill with spot putty until you have a nice, smooth finish.
Check the transition between the body filler and surrounding area to make sure you are smooth and strait. A good practice here is to use a “guide coat” to do this. Take either flat black primer or paint, and gently mist a very thin top coat over the area. Now, take your 320 grit paper on a sanding block and gently work the block over the area with just a few sanding motions. Any low spots will immediately be revealed as the sand paper will remove the guide coat quickly, but won’t hit the low spots. Once your area is smooth and level, you are ready to paint.
Once complete the repair is ready for the application of primer surfacer and topcoats. It’s very important to follow the paint manufacturer’s instructions for your specific brand of primer or paint.