There are 3 main reasons why you’d need to remove Bondo.
Either you’re restoring a car, fixing a previous repair, or have applied filler poorly.
Knowing how to remove Bondo quickly will speed up your projects and any reworks you may do.
The following methods are the quickest and safest ways of removing body filler or Bondo from a car.
Importance of protecting the substrate
Protecting the substrate is something that’s talked about a lot in the trade.
This means that you’re protecting the original material as much as possible.
You should always keep this in mind and avoid using methods that cause further damage to the vehicle.
As quick as they can be, avoid using harsh grits, grinding discs, or anything that causes sparks to fly.
Sparks are an obvious sign that you’re damaging and thinning the metal.
If you then go to use dent-pulling equipment or weld thin metal, it will most likely result in blowing holes.
The only way to properly fix thin metal is to replace it.
Removing Bondo from a project car/restoration
If you’re doing a project car or full nut and bolt restoration, you want the car to be perfect.
This is an accumulation of all your skill that’s going to be driven around and shown off.
There are 3 different techniques I’m going to recommend for removing Bondo from a resto job.
The first is using heat, the second is using clean & strip discs and the final method is using sandpaper.
When removing a big amount of filler, glazing putty or bondo from a car, you’ll find that you use a mix of all 3 methods.
Use heat to remove Bondo
Using heat to soften and/or crack the Bondo is the safest and easiest bet. If you get it hot enough then it’ll start to crack and blister off anyway.
The main concern while using this method is the panel warping.
Putting too much heat into a panel will cause it to warp and distort. So you can’t nuke a panel with heat and hope for the filler to fall off.
You may have to reshape the panel anyway, depending on how much filler is on it. However, less warpage is always better. Any time saved by overheating the filler could be used up shrinking and reshaping the metal.
All that you’ll need for this method is a cheap heat gun and a putty knife and/or hand wire brush.
You could also use a blow torch, it’s more efficient but it’s harder to control the heat with this method.
Simply heat the filler using the method of your choice and then scrape it off with your putty knife or hand wire brush.
It will help to keep moving around the car as you work. Doing this intermittently will help to prevent or at least minimize distortion.
You’re not going to be able to scrape every bit of filler off the car, but it will remove the bulk of it.
To get the remaining filler off you’re going to need to use another method such as a roloc disc or a DA sander.
Use a Scotch-Brite clean & strip disc
Scotch-Brite clean and strip discs are one of the handiest tools for technicians.
I use these every day for cleaning paint, filler, and sealer off of cars. They absolutely tear away at them without damaging the metal.
The main issue you face when using these discs is the cost. They’re not exactly cheap and can burn through them if you’re taking off thick filler or coatings.
What you’ll need for this is a roloc attachment that fits your chosen power tool. I use mine on a heavy-duty drill or my air pistol grinder.
Once it’s secure on your power tool all you need to do is run it across the filler and watch the filler fly.
This is a very dusty method of removing filler.
Make sure you’re wearing a dust mask and working in a well-ventilated area when possible.
It’d be best to use method #1 to remove the majority of the filler and then follow up with this one. Doing this would seriously decrease the amount of dust created.
Use a Sander
The traditional way would be to use a sander to remove the filler. This is considered the “safest” way.
Old-school technicians would slap a 40 disc on and cut the panel way back.
Nowadays you’d start at 80grit as this is way less likely to damage the material.
Modern cars are using thinner sheet metal than the classics. So thinning the metal makes it significantly weaker.
The problem with using an 80-grit disc is that it’ll feel slow, especially if it’s rubbish sandpaper.
Mirka & 3M paper is the paper you want to use.
Again, even with a DA sander, you want to pay attention to how much you’re heating the panel.
Prolonged periods of sanding will put some serious heat into a panel.
This could cause door skins to pop off of reinforcement and other panel warpage.
Finally, if you do use this method, make sure to give yourself breaks. It’s tedious and exposing yourself to vibrating tools for long periods can cause HAVs.
How to remove Bondo while it’s still wet
As an experienced technician or DIYer, you’re probably never going to remove wet filler after applying it.
It’s more likely If you have a new helper or are learning yourself.
The main cause of having to remove bondo while it’s still wet is not putting enough hardener in it. This results in the bondo taking forever to dry, or not drying at all.
At this point you want to scoop as much filler as you can off with a spreader.
Once you’ve taken most if not all the filler off, you should then use panel wipe on a rag for the more stubborn bits.
If you’re struggling to get the last little bits off. It may be worth running a DA over the surface to prep the area again.
Removing Bondo From Plastic
Removing filler from plastic is a little bit tougher than removing it from metal. You can’t use a lot of the abrasive techniques that you would on metal.
Instead you have 2 options.
Try and crack it, then scrape it off or use a DA Sander and invest a lot of time.
A lot of fillers will crack with a little flex. As a result they tend to crack on their own if they’re in high pressure areas on a bumper.
All you need to do to crack the filler is bend the bumper or plastic a little in the filled areas.
Some fillers are more “flexible” than others as they’re made for flexible areas but most will crack enough.
Once its cracked, it should be easy enough to get a screwdriver or putty knife to scrape it off.
The second and most used solution is to sand it back using a DA sander.
It’s recommended to start with a P180 disc, but, I’d use a 120 to speed up the process.
After you’ve removed the bondo either reshape & refill it. Or take a 180 disc to smooth the surface before priming and painting it.
You can’t go much more aggressive on a bumper otherwise you’ll end up damaging the substrate.
While taking filler off a bumper takes longer, you’ll find that most bumpers aren’t caked in filler. Whereas there’s a lot of quarters, doors and sills full of mud.