Fiberglass and Bondo are both commonly used in the body repair industry.
They both do the same job. Helping to fill in dents, scratches, and other imperfections in your metalwork.
Despite achieving the same outcome, they’re both very different products.
In this article, I’ll tell you when to use Fiberglass vs Bondo and vice versa.
Knowing everything about these products will help you decide when to use them, how to use them, and where to use them.
Table of Contents
What Is Bondo?
Bondo or body filler has been around for longer than I’ve been alive.
The two-part solution that’s so common today was first developed in the 1950s by a WW2 Veteran.
Bondo is made up of mostly talc and plastic, and when mixed with hardener creates the durable body filler we know today.
This allows you to fill and shape a panel very quickly since it takes 20 minutes to harden and is easy to rub.
Nowadays there are many different types of Bondos or fillers.
They’re all body fillers and do a similar job. They just have slightly different properties and uses.
What Is Fiberglass
Fiberglass has a completely different makeup from Bondo.
As the name suggests, it’s made up of fibers. Glass fibers to be exact.
Raw fiberglass cannot be used as a filler. However, it’s still commonly used in the industry.
Raw fiberglass is also known as Glass Reinforced Plastic and is used often in kit cars.
The other and most common place you’ll find it on modern cars is aftermarket spoilers, splitters, and diffusers.
In this article, we’re comparing fiberglass filler or fillers with fiberglass resin.
Again, they come in a can and are a 2 part solution that needs a hardener to cure.
It works in the same way as Bondo and achieves the same end goal.
How Fiberglass and Bondo differ
Despite being very similar products, there are some key differences.
Fiberglass is harder and more durable
Fiberglass is seen to be harder and more durable than Bondo.
While both can last for decades when use correctly, fiberglass is expected to last longer
When Bondo is hit, it cracks easily. However, the woven strands in the fiberglass filler make it stronger and less likely to crack upon impact.
If you’ve got a big gap between panels and need to build the edge up, fiberglass is better and much less likely to crack or split. This makes it perfect for building up gaps between bumpers and fenders.
In extreme weather, the metal on a car shrinks and expands. Bondo is no different.
The shrinking and expanding of the car panels or body filler can result in cracks.
This is even more likely if you’re laying on thick layers of bondo and working beyond maximum fill depths.
That said if you’re prepping properly and following the technical data sheets there’s no reason bondo won’t last forever.
Bondo is much easier to apply and rub
Because bondo is made up of talc and plastic, it’s much easier to work with. This is probably why body filler is much more common in shops where efficiency is rewarded.
You can mix body fillers with dolphin glaze and make it even thinner and easier to apply.
The same goes for fiberglass. I’ve mixed it with dolphin to make it slightly easier to apply, but it’s still harder than bondo.
Another downfall of fiberglass is the fact it’s much harder to rub.
Fiberglass and all metal body filler are the hardest fillers to rub (in my opinion).
If you’re intent on using fiberglass, you’ll have to get a good DA sander to take the head off it.
Either that or you fiberglass for your first fill and change to body filler when you’re closer to the shape you want.
Fiberglass is non-porous
One of the main reasons why fiberglass can outlast Bondo repairs is the fact it’s non-porous.
This means you don’t have to worry about moisture entrapment when using fiberglass.
Whereas Bondo is porous and cannot get wet.
The talc in the Bondo attracts moisture.
Having moisture in a repair can lead to the filler bubbling up.
It can also cause much bigger problems under the filler with rust forming beneath it.
You can see this a lot of the time when you strip back filler from a previous repair.
This is one of the reasons why body shops tend to replace panels with previous repairs.
This way they can actually guarantee the quality of the repair.
Fiberglass has a worse finish
While fiberglass is good for the first fill, it’s not really a finishing filler.
It’s a much rougher type of filler and while it can be smoothed off with fine sandpaper you’re better off with body filler.
The harder nature also means that it takes forever to rub down fiberglass with finer paper.
You’re much better off hitting it with a 40 or 80 grit and then applying bondo or glaze.
While most people will finish their repair using glaze it’s a must when using fiberglass.
How Body Filler & Fiberglass are similar
Despite having different properties, body filler, and fiberglass both share the same goal and are used in the same way.
Both 2 Part Systems
Both body filler and fiberglass are 2 part fillers. They’re useless without a hardener.
Forgetting the hardener will mean that neither of the fillers will cure and you’re left scooping it off with a rag.
Both fill dents
Body filler and fiberglass both fill dents.
They both have similar fill depths and offer great build.
Regardless of which one you choose to use, they’ll fill the dents and allow you to shape the panel to the shape you need.
I’d personally recommend using fiberglass as a first fill, and then body filler after, but that’s completely up to you.
They both do a great job and will help you to achieve the finish you want.
Is Fiberglass or Bondo better?
One isn’t necessarily better than the other.
In some shops, using fiberglass filler is considered to be “rough” or “hack” work. It really depends on where you’re working and the type of job you’re doing.
They both have applications where they excel and other areas where using something else may be better.
The fact that fiberglass is non-porous is great for filling panel joins or areas that have been welded and ground down. Whereas bondo is much better for efficiency.
They both have their uses and a shop that’s doing a wide variety of work will likely stock both.
It’s all about using each product and learning. This can be said for most if not all products and tools in the auto body industry.