Bondo

Bondo is a brand of two-part putty manufactured by 3M. Originally used mainly for automobile body repair, Bondo has been widely used by carpenters to repair wood. The Bondo brand of filler or putty is composed of 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 body filler, sand it to the proper shape, and prime and paint it like the material around it. The term "Bondo" is a registered trademark in the United States and in many countries throughout the world.

Bondo body filler 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 body fillers and other polyester body fillers are far safer to work with than their lead-based counterparts, they can still pose 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 body filler by walking around the car with a magnet, as the magnet will not be attracted to the body filler. Unscrupulous sellers may mix metal flakes into the body filler mixture before application in order to circumvent the magnet test. This test however, will not detect body filler on plastic, fiber glass, or carbon fiber components.