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).
In order to get started, you'll need a plastic spreader and a mixing pallet-a piece of cardboard will work, but a plastic mixing board or a scrap of plexiglass is better because it won't absorb some of the polyester resin from the filler. If you're new to the process, you may want to try a bit of a test before you get too far ahead of yourself.
A good way to get started is to scoop out a golf ball-sized wad of filler and apply it to your mixing board. Then, squeeze out about a 1/2- to 3/4-inch-long ribbon of cream hardener onto the scoop of filler. Using the plastic spreader, mix the two components together by spreading it back and forth and folding it over itself along the surface of the mixing board. Do this until the filler and hardener are thoroughly mixed and the color is consistent. You have to be kind of quick about it, because depending on the air temp you'll only have two to three minutes to work before the mixture starts to harden. Once you've got your first little batch mixed up, you can get a feel for it by trying to coat the surface of your mixing board as smoothly as possible. Give it a try and you'll get a real good idea of how the filler will act and how fast it hardens. If it hardened before you finished, then you used a bit much in the way of hardener. If 10 minutes have elapsed and it's still soft, then you didn't use enough. You'll get a handle on it with a little practice. It's worth a bit of time and material to practice before you start to slather it all over your project-and you will get a really good idea of how both the filler and you act when you're under the gun.
You never want to apply body filler to anything but bare metal. Applying it over any other substrate (like a layer of paint) means that its adhesion will only be as good as what's between the filler and the metal itself. Filler adheres to metal mechanically, not magically. In other words, the surface the filler is applied to should be roughed up with at least 80-grit paper. This creates scratches that the filler can "bite" into so it can hang on to the surface.
Givin' It A Try
Let's say you've got a door ding in the center of your door panel that you want to tackle. The first step is to wash the area with soap and water and then a swipe or two of wax and grease remover. This is to remove any traces of wax that might be on the panel so you don't sand it down into the grain of the metal and cause an adhesion problem. Next, sand an area two to three times larger than the dent you want to fix down to bare metal (remember, filler needs to be applied on a clean, bare-metal surface) using 60- or 80-grit paper or a grinding disc-again, this is to give the filler a rough surface to bite into and hold on. The reason for stripping an area so much larger than the spot to be prepared is so you'll be able to spread the filler over a larger area so the transition from filler to base metal will end up so gradual that it'll be as close to completely flat to the eye and hand as possible.
Once the damaged area is prepped, it's then time to mix up a small batch of filler. Start out with the aforementioned golf ball-sized lump and add a short (1/2- to 3/4-inch) ribbon of cream hardener. Take a Bondo spreader and thoroughly mix the two components together by swiping the mixture back and forth across your pallet, folding the mixture back over itself as you do so. Do this until the color of the mixture is completely uniform. Once thoroughly mixed, you're ready to apply your first coat. The first application is to fill the indentation itself. Bring the level of filler up to and perhaps a bit higher than the surrounding surface. Allow the filler you've applied to harden thoroughly (anywhere from three to 10 minutes). You can test the rate of cure by lightly touching the surface of the filler with the backside of your finger between your nail and first knuckle (for some reason, testing with a fingertip will often pull up a piece of uncured filler, leaving a divot in the surface).
Once you've determined the filler has cured, take a sanding block loaded with 80-grit paper and begin to run it back and forth over the surface of the filler in long, smooth strokes, making sure to extend your strokes out onto the surface of the panel well beyond the outer edges of your filler. At this point all you're trying to do is sand the filler down to as close to the surrounding undamaged panel as possible-no need for this step to be perfect, though, as this is where the next step of the process comes into play.
The next step is a second coat of filler. This is called a "skimcoat." The skimcoat is an application of filler that's about an eighth of an inch thick that you will apply to the panel over a much larger area than your first coat (depending on the situation, sometimes the skimcoat can be or is extended over the complete panel). With the skimcoat applied to a large section of the panel (it's OK if you've extended it out onto the painted surface of the panel or the complete panel at this point) and thoroughly cured, it's then time to begin another round of sanding using a sanding block or board. A rule of thumb in this case is the larger the filler area to be sanded, the longer sanding block or board you'll want to use. Again, 80-grit paper is a good starting point. Begin sanding the surface of the filler in long, smooth strokes that extend past the edges of the area coated with filler. You don't need to muscle the board; use a light touch and let the sandpaper do the work.
It's this sanding step of the skimcoat that will hopefully transform the area into a nice smooth section that'll blend into the panel surface itself. Use long smooth strokes and try your best not to sand any one area within the coating of filler-you want to evenly sand the complete filler surface. Sanding spots within the perimeter of the filler area will always cause you to create low spots that you'll have to attend to with additional applications of filler-a vicious circle you don't want to start.
With the skimcoat applied, cured, and slightly sanded (just enough to take off the skin of the cured filler), the next step is to switch to a finer-grade paper, say about 180-grit or so, and the application of a guidecoat. A guidecoat is a dusting of contrasting color applied to the filler's surface. I've always used a mist of dark-colored primer, though these days there are guidecoat products (both wet and dry) made specifically for this purpose. The guidecoat provides a contrast between high and low spots on the filler surface as you sand. When you sand over a guidecoat with a flat block or board (don't ever sand filler with handheld paper; the uneven shape of your hand behind the paper will always cause you to remove filler unevenly) the dark color of the guidecoat is sanded away on the high points and left as dark areas on the low ones, letting you know just where you stand in your quest for a smooth, flat surface.
Once you get to the point of a nearly flat surface, switch paper grades again to around 220-grit to minimize sand scratches and to finalize the shaping process by block-sanding the surface extremely lightly. At this point, a good wet coat of primer can be applied to the surface and, once dry, sanded lightly with 320- or 400-grit paper to finalize the repair and ready it for paint.
Working with body filler is like any of the other innumerable chores along the road to a completed project. But, unlike bolting on a component or welding on a tab or bracket, working with filler takes practice. Hopefully I've given you a bit of info and motivation here. Don't be afraid to give it a try; it's really not as hard as you might think-like I've said thousands of times in the past, if I can do it, anyone can.