The bumpers on most modern cars are manufactured from fibreglass. While fibreglass is a durable material that can bend a little and still return to its former shape, if it is hit hard enough it will crack. Fixing a fibreglass bumper is easier than fixing a metal bumper, however.
Remove the bumper from the car. Consult the workshop manual for your specific make. You will need to remove the bumper since you will be working from both sides.
Examine the crack to determine how large it is. If it is a small crack, you can apply a layer of Bondo to repair it. If it is a larger crack that needs to be fibreglassed, proceed to step 5.
Clean the area to be repaired with mineral spirits.
Mix the Bondo according to the directions on the package and apply it to the crack and the area about 5 cm (2 inches) around it.
Let the Bondo dry and then sand it down, starting with a rough grit (60 grit) and then moving up to smoother grits (as smooth as 400 grit).
If the crack is larger than 1.5 mm (1/16 inch), you will have to apply fibreglass weave to reinforce it.
Grind the surface of the bumper down a few inches to each side of the crack.
Clean the area thoroughly with a mineral spirits and a clean dry rag.
Mix up the fibreglass resin according to the directions on the box and apply it to the crack on the backside of the bumper and the area around it with a brush.
Lay a patch of fibreglass cloth over the resin, making sure there is enough coverage to give proper support to the cracked area.
Brush more fibreglass resin over the fibreglass mat until it is thoroughly saturated. Try to keep the resin and fibreglass mat as flat and level as possible to reduce the amount of sanding that will occur later on.
Allow the fibreglass to dry. Sand the repair down until it is level with the nearby area.
Use Bondo to fill any imperfections on the visible side of the crack. Sand with 60-grit, 150-grit, 220-grit, 400-grit and then 1000-grit prior to painting. Use a block for the sandpaper to ensure even sanding.
Wear the proper protective equipment when working with fibreglass and the chemicals involved. This should be a good respirator and gloves.