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Bondo is a two-part putty manufactured by 3M. While the term “Bondo” is a registered trademark, it is also commonly used in the U.S. as a genericized trademark to refer to all auto-repair putties or so-called plastic body fillers. Bondo is a polyester resin that when mixed with a hardener (an organic peroxide), or catalyst, turns into a putty which then sets and hardens. The user can apply the mixed Bondo, sand it to the proper shape, and prime and paint it like the material around it.
Automotive Bondo was developed as a replacement for body solder, or molten lead, that was formerly used for the same task. Body solder is much more durable, but may require more effort to apply. Lead-based solder poses toxic hazards for people and the environment due to the heavy metal components. Despite the fact that Bondo is far safer to work with than its lead-based counterparts, it still poses significant health risks. The fumes are quite toxic, and the hardeners can create burns in cases of prolonged skin contact. The use of gloves, a mask, and proper ventilation are all recommended when mixing and applying the filler.
When buying a used car, it is possible to check for damaged areas that have been repaired with significant amounts of Bondo by walking around the car with a magnet, as the magnet will not be attracted to the Bondo. Unscrupulous sellers may mix metal flakes into the mixture before application in order to circumvent the magnet test. This test however, will not detect Bondo on plastic, fiber glass, or carbon fiber components.