Bondo is a filler, and won't "bond" pieces together, so if the crack is flexable, the bondo may fill it, but if it flexes, the bondo will split down the crack. Bondo is pretty durable, so you can prime it, and paint right over it.
Basically, you mix part a+b and you have about 5-10 min to work with the stuff. You want to make sure you don't use too much of the accelerator (red stuff in the tube) or the bondo will cure too quickly. You final mix should be very light pink in color.. like a tanish pink. Usually it will come wil a little plastic spreader, that is the approximate color you want to achive in your mix. Once you have it mixed, just spread it on.
If the crack is deep, a few thin layers is better. You can slop it on, and waite for it to "gel" and trim off some of the excess with a razor or exacto blade befor it fully cures.
Depending on the material it is applied to, be careful sanding, because the bondo will be eaten away much more quickly then the surrounding area, and if you are too agressive, you may be left with dimples, or a crease that you would need to refill.
