For repairing the cracks, you need to detach the bumper from the vehicle. Consult the owner’s manual to see the bolts or clamps attached to it before removing it from your car.
Examine the cracks closely to see how deep and long they are. Smaller cracks can easily be dealt by applying a layer of Bondo and it will be as good as new. However, for bigger and deeper cracks, you have to follow certain repairing technique.
Begin with cleaning up the area around the crack with mineral spirit. Make sure there is no dirt or debris within the crack or on the surface around it.
Take sufficient amount of Bondo and mix it according to the instructions mentioned on the pack. Apply it on and around the cracked surface to a few inches.
Wait until Bondo dries completely. Then, sand the area, firstly with rough grit and then the smoother grit sandpapers. This will make the surface a bit rougher.
If the crack in the bumper is more than 1/16 inch wide, you will be required to use fiberglass weave for the reinforcement purposes.
Keep grinding both sides of the crack to a few inches.
Once you are done with the sanding, use mineral spirit to wash out the area again and clean it dry with a piece of soft cloth.
You will need the fiberglass resin for the next step. Mix the resin as directed on the package and apply it on the back side of the bumper. Cover the crack and few inches around it. An unused brush can be used for this purpose.
Take a patch of fiberglass cloth and place it over the resin. Ensure the cloth completely covers the cracked area.
Apply some more fiberglass resin over the cloth and try to keep the mat and resin as flat as possible, This will reduce the amount of sanding afterwards.
Allow the resin to dry out and then use the sand paper to grind the patch, levelling it with the surface of the bumper.
You can use Bondo to fill out any spots left and then sand it again.