To apply body filler, in car repair, begin by sanding and cleaning the area. There are various body fillers, and the consistency changes with different brands. Always think about spreading many thin layers, instead of applying one think one. Layers should not be thicker than 1/4".
Make sure that you mix the body filler well. Stir it with a stir. You should get in the practice of kneading tube products as well. You will need something to use as a painter's pallet. Scoop out a bit of it and place it on your pallet. For every golf ball size of body filler, you will need a dime size of hardener. Mix them together with your spreader. Try not to let air mix with it. When it is blended completely, apply it to the car. It will start hardening fairly quickly. You need to be finished with it in 5 minutes or less. Apply a very thin coat. When it dries, you will need to sand it.