Using bondo is something nobody likes to have to do. The stuff is an admittance in your failure of metalsmithing mastery. Still, Bondo is no wussy taskmaster on it's own. It requires a steady hand and a sense of surface space, gradient approximation, and speed - or more appropriately, the sense of balance between speed and perfection. Maybe most critical is the ability to gauge available possibilities with the stuff, and what requires more drastic measures. The roof came to me dented, the result of a mix of garageless car, high wind, and trees with weak branches, so I knew it would be a bit of work to fix it. The last owner knocked the dent out from the inside as best as possible, then slapped down a skim of Bondo, and then took a nap, never to return with a sanding block. Now we finish the job.
Though it's a simple substance, the two part epoxy we all know and love is often the bane of inherited project cars. Too many fall prey to its siren song of cheap and fast repair, and thus end up filling cavernous dents with the stuff. Fortunately, in this case, the repair area is not too bad, from a depth perspective. However, the damage is over a long distance, probably almost a foot... and on a car with strictly straight front to back lines... and on one of the few curved surfaces... and right over the prominently visible drivers door. Given that, this presents some unique challenges.
For those of you unfamiliar, get ready for a primer (ha!) on the art of Bondo. There are several levels of knowledge here, I am by no means a master, but I've picked up some tricks over the years. First and foremost, make sure the area of repair is clean and free of paint, rust, grease, cheeseburgers, primer, and dust. Feel free to get to that point in whatever creative or low budget way you like. When mixing, use some type of surface that is portable and will forever be dedicated to Bondo mixing. I've got a nice piece of plastic set aside for this. Lay down the gray filler in the middle of the surface and then squeeze setting agent out next to it. The proper amount of setting agent to add is determined by black magic, but it's almost impossible to do it completely wrong. Here is a good time to put your latex gloves on. Use your Bondo paddle(s) to quickly mix the two elements together to an even color.
Now get to work laying it down, move quickly because you've got about five minutes before things get sticky. Select a Bondo paddle appropriate for the size of the defect. With firm, even pressure push the Bondo into the valleys of the repair area. If you're paddle can bridge the valley, stand it on end while applying the Bondo so as to mimic the original profile. My personal rule is to never lay down more than an eighth of an inch of Bondo at a time, and never more than a quarter of an inch in total. If it looks like it will take more, that's why man invented body hammers. I like to take my time with Bondo, rushing it just gives you a crappy end result. Even though the can says you can start sanding within an hour, it usually results in Bondo clogged sandpaper. Give it a couple of hours to set up, there are plenty of other things to fix in the mean time.
Sanding is the yin to the applications' yang. Whereas applying is a hectic race against chemical reactions, sanding is about gradually reaching the zen state of surface perfection. As illustrated, I like to sand in the dark. Why? Well, who knows what evil lies in the peaks and dips of Bondo? The shadows know. Using a strategically placed light source allows you to see what your hands can feel. This greatly increases the efficiency of your efforts. Unless you are a glutton for punishment and wish to hand finish things, grab an orbital sander with about an 80 grit sandpaper. Again, take your time working things out, find the high spots and work until the surface is uniformly as smooth as you can get it.
With an area this big, you'll probably have to reevaluate things now. Use your spotlight to look at different angles of the repair. I like to have a pencil handy here to make notes on the surface. If you see a peak, draw its outline and a rough map of it's topography (I use plusses and minuses, bigger plusses mean higher peaks, smaller minuses mean shallower valleys, and vice versa). This will give you a map of where to sand next or where to apply your next skim coat. Another tip: For bigger jobs like this, I like to use the two different colors of setting agent, blue and red. It provides contrast to the different layers, indicates where things have been worked already, and shows you how the repair has evolved.
When you've got what seems to be a nice smooth surface to the hand, mask off a generaous area to prevent unwanted overspray and lay down a couple of thin coats of high build primer. I garantee you won't be finished after that unless you're some kind of bodywork Michelangelo. The primer layer usually does a great job at showing off defects for you to go after. Repeat as necessary.
The process can be tedious, and a pain the ass, but you can't go around gangster style with a dented sled now can ya?