What Is Bondo?

Body fillers have been used in garages for decades, but there’s a time and place for their application.

byKevin Williams| PUBLISHED Sep 5, 2022 3:30 PM
What Is Bondo?
Tony Markovich
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A car without Bondo will always be better than a car with it, but that doesn’t necessarily make the popular automotive body filler bad. In certain contexts, it can be quite the contrary. The name is synonymous with quick automotive body work, and it’s been a mainstay for garage repairs for decades. 

To form a proper opinion on the plaster-like material, it’s important to first understand what it is, what it’s used for, and why it became so prevalent. Is it overly hyped or overly hated? Is it a must-have product or one you should never use? Let’s talk about it, as Bondo is more complicated than you’d think at first glance.

What Is Bondo?

Bondo is the brand name of a polyester body filler commonly used in automotive body repair jobs. Similar to Kleenex, or The Club, the brand name has been genericized to the point that any brand of automotive body filler is glibly referred to as Bondo. Sometimes body repair people refer to automotive filler as putty, given its putty-like texture.

Automotive body filler is used to smooth out imperfections in surfaces, often after a minor accident or ding. Small imperfections that are impractical or impossible to rectify are smoothed over with body filler, not unlike how one would spackle a dent or small hole in the wall in a house or apartment. The body filler is not unlike an epoxy, meaning one part of the solution needs to be mixed with a catalyst, in this case a hardener. When the two parts are mixed, that will cause the filler to harden after being applied to the surface. From there, the filler can be sanded smooth, primed, and painted to match the rest of the panel. 

What Is the Correct Use for Bondo?

If a car is badly damaged or has excessive rust, the correct way to repair it is by removing that metal and replacing it with fresh metal, especially if it’s a valuable car. However, if the damage is on a small scale and/or the vehicle isn’t worth much or in good shape, Bondo is perfectly fine. When done correctly, and used sparingly, Bondo and other brands of body filler lead to a nice finish that should last quite a while. 

For most people, car damage is inevitable, and body filler is simply a tool that any automotive technician would use for restoration help. When used incorrectly, however, it can be a car owner’s worst nightmare.

When Is Bondo a Bad thing?

Body filler is a sort of finishing part used to fill in small gaps and dents or smooth out rough edges. It relies on the user having repaired the underlying structure within reason, using the putty to fill in the little sections that aren’t possible or practical to fix.

Yet, that hasn’t stopped lazy body techs from skipping the metalwork repairs and instead just slathering on body filler to make things look sort of correct. This is bad, for several reasons.

For starters, it’ll probably look horrible. Priming and painting the surface can probably match the reflective qualities of sheet metal pretty well, but making sure vehicle character lines and dimensions stay true to stock isn’t an easy task when dealing with acres of thick putty. 

Body filler also isn’t as strong as sheet metal. If excess body filler is covering up serious structural issues, body filler isn’t going to absorb crash forces. Body filler isn’t necessarily a rust inhibitor, either. If rust damage hasn’t been repaired and the rust cut out or coated with an actual rust inhibitor, slathering body filler on top won’t do anything but hide the damage. That rust will continue to spread, but unseen, underneath layers of mud and crap. When it does finally show up down the road, it’ll be a much larger disaster.

Furthermore, even if body filler has been applied correctly, numerous body filler spots all over the car could indicate that the vehicle’s had a rough life. 

How Do I Find Out if My Car Has Any Bondo?

Body filler, when applied and finished correctly, is often imperceptible to the common person. Still, body filler isn’t thin like metal, and there will be differences in material thickness along body panels that have been filled in with Bondo.

Back in the day, it was common to use a magnet to find out where body filler had been applied. Wherever the magnet didn’t stick to the metal panel, that means some sort of non-metallic body filler had been applied. These days, that’s not always the most reliable way to decipher body filler. Modern cars are made of many types of metals, some of which don’t respond to magnets. Other times, if the body filler is thin enough, the magnet will work just fine.

The best way to locate body filler on a vehicle is to find the density of certain spaces. Body filler is a lot denser than metal. Electronic tools made for detecting body filler do so by measuring surface density. Heck, some enterprising handymen have taken to using electronic stud finders.

Some swear by the “knock test,” simply knocking on a body panel to determine its sound, but that’s not always a surefire way to determine bondo, especially with new vehicles that can use composite manufacturing materials or plastics. A metal panel might not quite sound like metal these days, y’know? On older cars, though, this test may be useful. 

How Long Does Bondo Last?

When applied correctly, Bondo, or any other body filler should last the life of the vehicle. When applied incorrectly, like, on top of corrosion without any rust inhibitor, the bondo won’t last all that long. Excessive moisture, or improper technique with regard to mixing or application can cause bondo to crack, and break. For example, Youtuber EricTheCarGuy did a cheap fix on a rusty Acura Vigor. He was adamant that his job was hasty and quick, and thus after about two years, his bondo patch job has already started to bubble and crack.

Remember, Bondo and any other brand of body filler is simply a tool to recondition a broken car back to good condition. When used correctly, it’s great. When used incorrectly, it can be an enthusiast’s worst nightmare.