Bondo and fiberglass are both excellent auto-body filler materials. If you have a dent or scratch on your car, either Bondo or fiberglass can be used as a filler and smoothen the finish.
Both do a great job of concealing unsightly marks on your car’s surface. However, they are not the same. In some areas, Bondo outshines fiberglass and vice versa.
If you are looking for the best filler material, here is a review of Bondo versus fiberglass.
What is Bondo?
This is a two-part putty compound that can repair scratches, rust, holes, dings, and dents on a car’s finish. Bondo conforms easily, and that’s what makes it a good filler.
It cures fast, and the manufacturers claim that Bondo doesn’t shrink. If applied correctly, Bondo can offer lasting results. Other than metal, it can also be used on wood, fiberglass, and concrete.
Bondo has been the go-to filler for many car owners for decades.
Bondo was created in an era where auto body repair entailed pouring heat shrinking metal over dents and scratches.
Just from the description of what folks used to do back in the day, that involved a lot of work. Also, it’s something that had to be handled by a professional considering how dangerous handling hot shrinking metal is.
Anyway, thanks to scientific advancements, Bondo was formulated. It is popular amongst both DIY enthusiasts and professional detailers. This two-part putty comes with a hardener.
The latter is responsible for ensuring the material turns into a durable substance. When mixed, the Bondo spreads easily into scratches and dents. Please note that it doesn’t harden right away.
Applying Bondo on a surface is a relatively simple procedure. You need an applicator pad and use the correct ratio of hardener and Bondo. The time it takes to harden is around 25 to 30 minutes.
Bondo is made by 3M, a company that has gained recognition in the auto industry for making quality products. Since it was developed in 1955, Bondo has been used on thousands of cars. And people have had varying experiences with it, both positive and negative.
✔ Fast and Reliable Repairs – Perfect for filling dents, rust holes, dings, and scratches, this original formula body filler is ideal for DIY and professional restorations.
✔ Quick Curing Time – Mixes easily with the included red cream hardener and dries in just 20 minutes, letting you sand and paint the same day.
✔ Smooth, Non-Shrinking Finish – Lightweight, spreads evenly, and won’t shrink or crack—great for prepping panels before paint.
✔ Versatile Surface Compatibility – Works on steel, aluminum, fiberglass, wood, and even concrete—not just for cars.
✔ Visual Mixing Guide – Salmon color appears when filler and hardener are fully blended for confident application.
What is Fiberglass?
Nowadays, fiberglass has grown in popularity, and it has many applications in various industries. It is a soft fabric-like material that is made from woven glass. Raw fiberglass can’t be used as a filler.
However, when saturated with liquid resin, it hardens into a filler that can cover dents, and scratches and perform essential repairs on the car body.
Even though it is not as popular as Bondo, fiberglass has become a top-choice filler because of its durability. Bondo, which is known to fall off after a while, fiberglass is a more durable alternative. There is a big difference between fiberglass filler and raw fiberglass.
Fiberglass filler comes in a can, and for it to work, it should be mixed with a hardener.
Fiberglass filler mixes just like Bondo does. However, it is thicker and therefore more difficult to mix. The reason why it is harder to combine is the presence of fiberglass. It also comes with a hardener which ensures fiberglass sticks to the car body.
Other than durability, detailers also love fiberglass because of its waterproof properties. Because of its nature, fiberglass doesn’t attract moisture, nor does it absorb it.
Car owners who want to prevent rust should opt for fiberglass instead. They are harder to sand. Brace yourself for more elbow grease. But because it’s stronger, it should serve you for longer.
✔ Twice the Strength – Packed with fiberglass strands and fibers, this filler is 2X stronger than standard body filler for repairs that hold up under stress.
✔ Fills Without Backing – Covers holes up to 1/2 inch with no need for backing strips, fiberglass cloth, or resin.
✔ Solid Base for Full Repairs – Excellent as a foundation layer for rust-outs, dents, and damaged panels—follow up with body filler and glaze for a smooth finish.
✔ Compatible with 3M Body Repair System – Designed for use in the Fill stage of your bodywork process.
✔ Includes Cream Hardener – Comes with red cream hardener to activate and cure the filler properly.
How Are They Different?
-
Fiberglass is More Durable than Bondo
The first key difference between these two is regarding durability. Fiberglass is way stronger than Bondo resin. Therefore, when you use fiberglass filler, expect it to outlast car parts that have Bondo.
Once they harden on a dent or scratch, the strands of woven fiber should hold on for years. Fiberglass will resist shrinking, the effects of the weather, and other elements.
A well-done fiberglass job should stick for as long as you will own the car. Bondo, on the other hand, we can’t say the same. Bondo fillers have been known to shrink and fall off easily.
If you want Bondo to last long, you must apply it correctly. However, if you do a mediocre job, it will fall off in a few months.
-
Bondo is Easier to Mix and Apply
These two materials have varying physical compositions. As a result, one mixes more effortlessly than the other. In this case, Bondo is the easiest to mix.
The two-part putty resin mixes pretty easily with the hardener. It shouldn’t even take you more than two minutes for you to get the Bondo mixture ready.
Moving on to fiberglass, it’s not that easy. First and foremost, fiberglass is thicker. It, therefore, requires a lot of effort to mix with its hardener.
And while we are still on ease of use, fiberglass is harder to work with. Because of its thickness, it will take you more time to fill the fiberglass filler inside the scratches and dents. You have to be very patient when using fiberglass.
-
Fiberglass is Waterproof, Bondo Isn’t
One of the drawbacks of using Bondo is that it causes rust. It contains talc, which attracts moisture, and that’s a recipe for rust.
When you Bondo your car and it happens to fall off, you are most likely to find rust spots if you look at the dent left behind.
If you had used fiberglass, you wouldn’t have to worry about rust because this material doesn’t attract moisture.
It is waterproof. If the metal beneath is rust-free, fiberglass will ensure it remains that way. And because fiberglass is durable, it will protect your car body from rust for very long.
-
Fiberglass is a Bit More Rigid than Bondo
Because of its thickness, fiberglass is relatively rigid. And that’s why it is tough for you to mix it with hardener. This makes the application process even harder when using fiberglass.
Bondo is very flexible, and its application is super easy. It conforms to the shape of the scratch or dent. As long as you have an applicator pad, the filler should spread easily.
How Are They Similar?
Both;
-
Do a Great Job
If you want to perform auto body repair, whether you go with fiberglass or Bondo, you can be assured of great results. Both are excellent fillers.
You will be able to fill scratches, dents, and dings on your car without engaging in costly repairs. It is important to note that despite both being good fillers, the decency of the final result depends on how properly you apply it.
-
Require a Hardener to Stick to the Metal
Whether you settle on fiberglass or Bondo filler, you must use a hardener. These products turn the filler from the soft substance into a hard material that will stick to your car. Hardeners work after a few minutes.
The reason for this is to give you some time to spread the filler into the spaces on scratches or dings. As you shop for body filler, ensure that you get a hardener as well. The good news is that most manufacturers package both together.
What’s Better about Bondo?
Bondo is an excellent filler appreciated by both professional and DIY detailers. It doesn’t have such an excellent reputation amongst DIY enthusiasts since most of them apply it incorrectly. However, if you ask experts, they will tell you that Bondo is the best.
That aside, Bondo is a very flexible filler material. It is easy to mix and apply. You don’t even need special skills to spread it on your car.
When you compare it to fiberglass, Bondo doesn’t need a lot of effort when it comes to application. Fiberglass takes longer to apply because it is thicker.
It isn’t easy to sand. You won’t have to worry about these issues when using Bondo. Also, Bondo is a trusted name in the auto industry. You are therefore assured of quality.
What’s Better about Fiberglass?
Between these two materials, fiberglass is the strongest. Bondo has a history of shrinking and falling off easily. On the other hand, fiberglass is quite durable, and if applied correctly, it can last for years.
This filler is also waterproof. A feature that you can’t get from Bondo. Fiberglass may be a bit difficult to apply, but it is an excellent choice when you consider the perks that it offers.
Fillers made using fiberglass are sold by various companies and brands. It is essential to do some research before buying a fiberglass filler. This is to ensure you get the right thing.
Who Should Get Bondo and Why?
Go for Bondo if you are the kind of car owner who prefers a filler that is easy to use. If you also have a thing for mainstream products, Bondo is a name that has been around for years.
Who Should Get Fiberglass and Why?
Fiberglass should be your top choice if you want a more durable filler that doesn’t let water in. Fiberglass filler is gradually becoming popular as an auto-body filler. The only problem is that it’s thick and doesn’t conform easily.
Was This Article Helpful?
did not explain how to mix bondo glass and hardner,
Hi Loyd! Thank you for your feedback. To mix Bondo Glass and hardener, you generally need to apply a small amount of hardener to the Bondo Glass resin. A good rule of thumb is to use about a 3-inch diameter circle of resin to a 3/4 inch line of hardener. Mix thoroughly until the color is uniform. Remember to work quickly once mixed, as the material will begin to harden. If you have any more questions or need further clarification, feel free to ask. We’re here to help!