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).