Body filler, also known as bondo, is a type of putty used to fill in dents and holes in car bodies. Although bondo is strong and durable, it can crack if not applied correctly. There are several reasons why body filler might crack, including:
Not mixing the hardener properly - The hardener is what sets the body filler and gives it its strength. If it's not mixed properly, the bondo will be weaker and more susceptible to cracking.
Applying too much pressure - When sanding body filler, you need to apply even pressure. Otherwise, you risk creating weak spots that are more likely to crack.
Not letting the bondo cure fully - Body filler needs time to cure before it can be sanded or painted. If you try to rush the process, the bondo might not be as strong and could crack under pressure.
If your body filler does start to crack, don't panic! You can usually fix small cracks by sanding them down and reapplying fresh bondo. Just make sure to mix the hardener properly and give the new layer of bondo plenty of time to cure before sanding or painting.