What Exactly is Bondo?

What Exactly is Bondo?

If you enjoy talking about cars, particularly old, used cars, you are going to hear the word “Bondo” at some point.  Bondo is an auto body filler that has been used for over 60 years to repair automotive sheet metal panels. It sometimes gets a bad rap because it can be applied poorly which results in finish that fails quickly. We asked the auto body tech at Kims Toyota of Laurel, a local Toyota dealer in Laurel, MS, to explain this auto body product a little more.

Before Bondo

Before the 1950s, auto body repair was done using lead solder. Lead solder was an excellent body filler but it was quite difficult to use.  Here’s how it was applied: First, a damaged panel had to be hammered out and warped sections had to be heat-shrunk until it was “roughly” flat. And, this required considerable talent to do properly. After the panels were roughed in, they were heated up with a torch and lead solder would be melted on the damaged area. Once the area was coated with the lead, it was filed down and then sanded. Although lead was a great body filler (it couldn’t rust), lead was dangerous not only because it took a torch to apply (hot, liquid lead) but the fumes that were emitted from the molten lead were toxic.

Plastic fillers

In the 1950s, Dupont released a polymer-based auto body filler.  The filler was marketed as “Bondo” and the name today has become a generic word for all plastic body fillers. Bondo itself is a complex mixture that contains, among other things, fiberglass (polyester) resin and talc. The resin allows adhesion as good as most epoxy adhesives (which is excellent) and the talc (as in talcum powder) makes the material flow smoothly.

Bondo uses a catalyst to harden. It is a two part substance where a catalyst (usually MEK peroxide) is mixed into the plastic filler. Anyone who has ever worked with Bondo knows that MEK is an aromatic chemical compound that is dangerous to one’s health. Bondo cans are covered with written warnings that tell you to use the materials in a ventilated area and to use a proper respirator.


When used properly, Bondo is an excellent product. It hardens rapidly so body work can be completed quickly. Once a body repair person gets the hang of it, the product can do wonders filling small dents and dings in auto body panels and gets a car in and out of the repair shop quickly.

Bondo isn’t perfect,though. One of the main components of Bondo is talc (like talcum powder) and talc is hygroscopic (that is, it absorbs moisture). Bondo thus is best where the metal is solid, in other words, there are no holes behind it. If there are holes or rust under the Bondo , it will likely lift in a few years.

Another issue is that the temperature that you use Bondo in is critical.  Curing is drastically slowed below 64 degrees F ambient air temperature. The best temperature to use filler is between 70-80 degrees F, and that includes the metal you’re applying the filler to. If the metal is cold, the mixture will cure from the outside in, trapping moisture against the metal surface.

Backbone of the industry

Bondo is a major product for the world’s body shops. It is made by dozens of manufacturers today and it is inexpensive. Bondo does get a bad rap sometimes. Some do not think it is a quality product for extensive body repairs. They actually couldn’t be further from the truth. Bondo will last for decades if properly applied to a clean solid surface. The problem is many body repair technicians use it to fill holes and rust. When Bondo is used like this, the product will soak up water and fail quickly.

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