Before I get rolling, I want to clear up one thing: Bondo is a brand name, though it's commonly used as a general description referring to any and/or all body fillers-kinda like Kleenex is to tissues. With that said, and with deference to the Dynatron/Bondo Corporation, I'll more than likely continue the aforementioned practice here.
What Is It?
Bondo is a polyester resin product that when mixed with a hardener (an organic peroxide) or catalyst, turns into a putty-like filler that then chemically cures and becomes rock hard. Its claim to fame is that a user can apply the creamy concoction of mixed Bondo to a dented automotive panel, sand it to the proper shape and smoothness, and then prime and paint it as you would with the metal around it.
Bondo was developed as a replacement for body solder, or lead, that was formerly used for the same task. Lead is more durable, but requires much more skill and effort to apply (or paddle, as they say), plus, lead poses nasty toxic hazards to people and the environment. Despite the fact that Bondo is far safer to work with than its lead-based counterparts, it still poses some health risks of its own. Bondo is based on unsaturated polyester resin, minerals, and glass micro spheres (tiny hollow balls). The normal formulation of an average lightweight body filler is something like this: unsaturated polyester resin (flexible putty resin), titanium dioxide (pigment), talc powder, fumed silica, and glass micro spheres. The filler is cured chemically by mixing it with a catalyst or hardener made of dibenzoyl peroxide cream (not the kind you put on pimples, though).
Basically, Bondo was developed as an answer to the dangers and skills required to perform the body repairs formerly allowed by the use of slightly melted lead. Just keep in mind, though body filler is much safer to use than lead, the fumes from filler's polyester resin bases are still toxic, and the hardeners can create burns in cases of prolonged skin contact. So naturally the use of gloves, a mask, and proper ventilation are all recommended-as are dust masks when sanding.
Why The Bad Rap?
Bondo's detractors (many whose opinions are based on the false notion that anything with filler on it is less than desirable) are motivated by the folklore provided by years of grossly misapplied body filler. Unfortunately, the introduction of Bondo and its ease of application supported the growth of a league of hackers who not only used Bondo for the repair of minor shallow dents and dings, as it was intended to do, but also used it to reform completely crumpled panels and to bridge huge gaps in metal left by rust damage. Since body fillers are, for the most part, pretty porous, they do absorb some moisture, and all these types of hackers are doing is hiding the hole and helping it to grow larger. Another no-no is excessive thickness. Those same hackers think nothing of slathering damaged panels with filler, disregarding the fact that you really shouldn't use more than a 1/4-inch layer at a maximum-hence body filler's nickname, "bodyman in a can."
Body Filler BasicsNo matter what brand of body filler you choose (I prefer lightweight filler; it seems to me that it spreads smoother and sands easier), be sure to actually read the instructions and follow them-especially the guidelines on the amount of cream hardener (get carried away and you'll end up with a big, hard turd on the end of your spreader that'll be useless for anything other than a paperweight). Also, keep in mind that ambient temperature really affects curing time (the hotter it is in the shop or driveway, the faster the filler will harden).