Whether it’s a metal panel or plastic, if its hit with enough force then the damaged area could crack. There are different severities of cracks you can get, which one you could get depends on the force of the impact and what panel of your car takes the impact.
Most cracks on cars occur on the front or rear plastic bumper sections. As the panel is hit the plastic folds inwards until it hits a bracket or other hidden structural component then the plastic splits in two. Sometimes the crack might be small and within the panel while other times the crack might be severe enough to split the plastic panel from top to bottom.
On metal panels it’s the paint that cracks and not the panel. This type of damage on metal panels occurs mostly on the edges of doors where the door might have hit a solid object like a wall, lamppost or another car. The impact splits the paint and severely dents the panel.
Another cause for the paint to crack could be a bad dent repair. Some SMART REPAIR services can by accident or by lack of training and experience crack the paint while pushing to hard and in the wrong place while trying to remove a dent from inside the panel.
The best crack repair method on a plastic bumper would be to plastic weald it. Some SMART REPAIR services would try and fill or even staple the crack, this would always result in the crack reappearing and leaving you with yet another repair to get fixed. With a plastic weald you are joining the bumper back together with the plastic itself which gives the panel its structural integrity back and makes it impossible for the crack to reappear.
The only 100% repair method that should be used on a paint crack on a metal panel would be to remove the effected paint down to bare metal until all the cracking has gone, then build the layers back up again finishing in a localized respray.