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Bondo was originally created to fix minor dings and dents in metal automotive panels, allowing these small issues to quickly be filled, sanded to the proper shape and smoothness, then primed and painted to blend with the metal around it. However, over the years this product has come to be seen as a do-it-all repair material that can be used on just about any surface.
The simple facts are that if you use a product on a surface or for a purpose which it was never intended, then the results are naturally going to range from poor to terrible. I have found, however, that when you properly use the right Bondo product based on the type of material and project, rather than simply grabbing the original automotive body filler formula for everything, using Bondo can be a quick, effective option for a wide variety of repairs.
What Is Bondo and How Does It Work?
Bondo is often lauded as an all-purpose putty, but it actually is a polyester-based body filler and cream hardener, which when mixed together, create a chemical reaction that begins to cure the mixture and make it chemically bond to the surface to which it has been applied. Depending on the type of Bondo product, I have found that it can be used to repair dents, scratches, small holes, and other minor surface damage on metal, wood, fiberglass, concrete, and masonry.
Since Bondo uses a chemical curing process, similar to super glue, it doesn’t shrink as it cures and hardens, offering a significant advantage over similar types of fillers. The result is a strong, semi-permanent patch that can be sanded to the desired shape and finish before being primed and painted. I use this product for cosmetic fixes only, as it is not intended to support the weight of the surrounding materials. If any serious damage needs to be fixed, it’s best to repair the damage with a more standard method, rather than attempting to use filler as a fix-all product.
Why Don’t All Pros Like Bondo?
If you don’t take the time to research Bondo, then you may be tempted to assume that a singular Bondo product can be used everywhere and for just about everything. I can assure you that this is factually incorrect. Like any other product, Bondo should be used for the purposes it is designed for and nothing beyond those purposes if you want the best results.
I have found that most pros who take issue with Bondo have failed to use the right type of Bondo product for the job, are using Bondo improperly, or simply prefer more permanent fixes over the quick fix offered by Bondo. For instance, if I were to use a Bondo Automotive Body Filler to fix exterior wood paneling, the repair would likely fail, but if I used Bondo Wood Filler, while following the appropriate steps for preparation and application, then in most cases I would be pleased with the aesthetic repair.
However, it needs to be noted that Bondo, like all other fillers, is not a structural product. It is not designed to hold weight, so if you are attempting to use it to support the existing material, then the project is doomed to fail from the beginning.
When Should You Use Bondo?
Similar to glue or tape, whenever I use a filler I take the time to note the material I will be working on and find the product that has been designed for use with that material. Bondo has several different types of fillers that can be used for a wide variety of applications, but if you attempt to use the wrong type of filler for the material, then you’re bound to be disappointed by the results. I use Bondo when I need to repair minor dents, scratches, small holes, and other types of cosmetic damage.
- For automotive repairs, you can fill and smooth dents, scratches, and minor rock chips with the Original Bondo Body Filler.
- For interior and exterior wood repairs, you can fix small holes, knotholes, and cracks to refinish wood furniture, patch fences, repair frames, and fill other wood surfaces with Bondo Wood Filler.
- For small holes up to ½ inch wide in fiberglass, metal, wood, concrete, and masonry, you can use Bondo Short Strand Fiberglass Reinforced Filler.
- For holes up to 1-inch wide in fiberglass, metal, and wood, you can use Bondo Long Strand Fiberglass Reinforced Filler.
- For sealing small cracks and holes in water-prone environments, like tubs, showers, sinks, or boats, use Bondo Liquid Resin.
When Is Bondo a Bad Idea?
While Bondo does have a wide range of uses, there are many times when this is not the right product for the job. I have seen DIYers and even some pros attempt to use Bondo for projects it simply isn’t designed to handle, such as applying Bondo to help support a damaged porch column or using Bondo to sculpt an entire missing section on the front end of a car.
Bondo is a filler. It should only ever be used for small cosmetic fixes. Never use Bondo as a structural support supplement. The largest hole or crack Bondo should ever be used to fill or fix is up to just 1 inch wide, and that is only if you use a Bondo product specifically designed for this degree of added strength and adhesion. Generally, most Bondo products should not be used to fill anything thicker than about ¼ inch, unless otherwise specified by the manufacturer.
Additionally, it needs to be noted that Bondo is a quick fix filler product. Whether you are using wood filler, body filler, fiberglass reinforced filler, or any other Bondo filler, this product should not be considered a permanent fix when compared to more conventional repairs, like cutting out the damaged area, patching it with new metal or wood, then using the filler to correct any minor cosmetic issues at the edges of the patch.
How To Use Bondo Correctly
Knowing which Bondo product to use and when to use it is only half the battle. You also need to know how to use Bondo properly if you want to achieve the desired results. As with other adhesive products, I find that it is essential to know exactly how to use Bondo before attempting to use it because you have a limited amount of time after mixing the hardener into the filler before it cures and hardens. Follow these steps to learn how to use Bondo properly, but always make sure to read the label of each Bondo product for any minor differences in the instructions.
- Prepare the area by sanding and removing any loose material, grease, oil, or dirt, otherwise the filler will not adhere properly.
- Mix the Bondo filler with the hardener by kneading it together until a uniform color is achieved. Read and follow the directions provided by the manufacturer for the correct ratio of filler to hardener. Only mix what you can use in about 10 minutes to prevent the filler from hardening before you can apply it.
- Apply the filler to small holes, dents, or minor imperfections using a spreader or putty knife, then wipe the filler as smooth and level as possible without removing too much filler. It’s always better to have a little too much remaining that you can sand off than having to mix up another batch of filler because you still have a low spot.
- Allow the Bondo filler to cure for 15 to 30 minutes. Always check the directions provided by the manufacturer to know exactly how long to wait.
- Sand the Bondo filler to the desired shape, then prime and paint, or finish as desired.
