Why Does Body Filler Crack? Understanding the Causes

Body filler is a commonly used product in the auto body industry to repair dents on vehicles.

Using a mix of polyester resin and a hardening agent, body filler can be spread and shaped on vehicle panels. Once hardened, it can be rubbed with sandpaper returning the car to its factory shape.

While body filler is a convenient and effective way to repair damage, it is not without its flaws. One of the most common issues with body filler is that it can crack over time.

There are many reasons why body filler may crack. Most of the time it is because of improper use of the product.

Providing you follow the data sheet and the instructions on the tin, the body filler should never crack.

Causes of Body Filler Cracking

As an auto body repair technician, I have seen body filler crack on many occasions. In this section, I will tell you the most common causes of cracking.

Inadequate Surface Preparation

One of the most common causes of body filler cracking is poor surface preparation.

Before applying body filler, it is essential to ensure that the surface is clean and dry. You also need to make sure the metal is rust-free and all holes are welded up.

Failure to do so can result in poor adhesion and eventual cracking or bubbling of the filler.

This is a result of moisture finding its way in behind the body filler via pinholes and other defects.

Improper Mixing Ratios

Body filler is a two-part system that requires the correct ratio of filler and hardener to be mixed well before application.

Another common cause of body filler cracking is improper mixing ratios.

Most fillers have a 2% mixing ratio, however, you can play a bit fast and loose with this.

It’s not uncommon for techs to add more hardeners during the winter and less during the summer. This helps to manipulate the work time.

While it’s not a precise art, you can normally get away with this.

I wouldn’t recommend this if you’re new though. They’re experienced journeymen and the majority have been doing it for tens of years.

I’ve only really seen improper mixing ratios lead to problems in extreme cases.

If you put in too much hardener the filler may dry too brittle and crack. It’s also possible for the hardener to bleed through the paintwork after it has been painted.

The opposite can also lead to problems.

Too little hardener can lead to filler not drying all the way through. This can result in cracking and bubbling as well.

Excessive Filler Application

Most body fillers come with a maximum fill depth.

The idea is that you get the metal as straight as possible before you even think about using body filler.

Applying too much body filler can result in excessive heat buildup which can cause the filler to crack.

Most technicians think the more filler there is the more likely it is to crack. It also depends on the location that you’re filling.

Using filler on the edge of panels and door jams to make gaps smaller is a recipe for disaster.

Car panels have a certain amount of flex and movement to them. As they move, the filler is likely to start cracking.

The same goes for impacts. If you’re punching or bending a door or bumper to make it fit, there’s a good chance you’re going to crack the repair.

Using The Wrong Filler

While back in the day there were very few types of body fillers, nowadays there are loads.

Body filler is a thermoset. It dries brittle. This means it’s prone to cracking and snapping when flexed.

There’s no reason to be applying traditional bondo to bumper covers. I’ve been guilty of it myself, and I’ve seen many journeymen also do this.

What you want to use when repairing bumpers is a flexible filler. There are plenty of plastic fillers that have flexible properties.

This means when a panel flexes or a bumper is impacted, it’s less likely to crack.

As long as you try to minimize all of these factors, you should be fine. You may get the odd occasion where it cracks but it’s going to be very very rare.

Effects of Environmental Factors

The environment can have some effects on body filler, but it’s very rare.

Temperature Fluctuations

While I’ve seen many people talk about temperature fluctuations, it is highly unlikely to be extreme enough to crack filler.

You’d need to have the car sit in extreme heat, only for it to quickly turn freezing cold. The metal does indeed expand in heat and shrink in the cold but it’s normally not enough to crack filler.

Moisture Exposure

As discussed earlier, moisture finding its way under the repair is going to cause issues.

So any exposure to moisture is going to be a problem.

However, this isn’t going to be an issue if the car is repaired properly.

Keep the lid on your filler as much as possible and store it in a dry area.

Make sure the surface is prepped correctly and your filler is mixed thoroughly. Wait for it to dry and harden properly before rubbing it and finishing with 180 paper.

Providing the car is repaired, primed, and painted properly the repair shouldn’t crack unless it’s damaged again via an impact.

Prevention and Best Practices

As with any auto body repair, proper application techniques and materials are key to preventing body filler from cracking. Here are some best practices to follow:

Proper Application Techniques

When applying body filler, it is important to work quickly and efficiently. This means mixing the filler and hardener thoroughly and applying it to a clean, dry surface.

Use a spreader and apply the filler in thin layers.

Build the depth as necessary until the desired thickness is achieved.

Avoid overworking the filler, as this can lead to air pockets and weak spots.

Once the filler has been applied, allow it to dry completely before sanding. This typically takes about 20 minutes, depending on the temperature in the shop.

Sand the filler with 80 to 180 grit paper until you’ve achieved the desired shape.

Use of Quality Materials

Using high-quality materials is also important for preventing body filler from cracking.

This includes using a high-quality filler and hardener, as well as a clean mixing board and spreader. As long as it’s a reputable manufacturer and it’s not past its expiration date.

Avoid using cardboard or other porous materials for mixing. This can introduce moisture, and unwanted debris and affect the curing process.

It is also important to use a filler that is compatible with the type of repair being performed. For example, a flexible filler should be used for repairs on areas that are prone to flexing, such as the roof or fenders. Using the wrong type of filler can lead to cracking and other issues down the road.

By following these best practices, you can help ensure that your body filler repairs are strong, durable, and free from cracking.

It also means that you’re not going to have any reworks coming back into your shop.