Bondo is Never the Answer

By Scott Sidler October 26, 2015

I was supposed to be posting on plaster textures this week but something happened this week that got me so riled up that I had to delay the plaster post.

how to patch wood with bondo
This column was patched with Bondo around the base. Already failing after only 2 weeks.

[Stands on Soapbox]

Bondo.

Bondo is a great product for patching damage to cars, but somewhere, sometime in the past some painter or carpenter decided that it would work just as well on wood. Let me be clear…Don’t patch wood with Bondo!

I could end this post there, but then I’d be asking you to believe me blindly and like most people I prefer to see some evidence before I’m going to accept new information as fact.

Here’s the deal: auto body filler was designed to fill holes in a steel car chassis not wood. That’s what wood filler was designed for. These are two very different materials and they require two very different types of filler.

If you want to read all about what types of wood fillers I recommend read my post The 7 Best Products to Patch Wood.

 

The Problem With Bondo

Like I mentioned before, auto body filler is meant to behave much the same way as the metal chassis it is used to repair. Car bodies are very rigid structures that don’t have much, if any, expansion and contraction due to weather conditions. Accordingly, Bondo is a very stiff product when cured which makes it well designed for auto bodies.

That same rigidity makes it a terrible choice to fill wood which can move a lot with seasonal changes due to temperature and moisture content. To fill wood you need a product that is specifically designed to bend and flex the way wood does.

Just like caulk is too flexible to effectively fill large gaps in wood, Bondo is too stiff to do the same.

When Bondo (or any auto body filler) is used to patch wood it fails in a very short time. It may take a couple of years at the longest or in the worst case scenarios I have seen it fail in a matter of weeks.

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The Science of Wood Movement

If you’re interested in knowing the science behind what happens I’ll lay it out very simply here. If that sounds boring then skip to the next section.

Wood naturally expands, contracts, and moves depending on the weather because all those cells that used to move water up and down the trunk still remember how to do their job. When water is introduced, be it rain, dew, or humidity the cells in the wood absorb the moisture and begin to transport it through the rest of the wood just like when the tree was alive.

The addition of moisture causes the wood to swell, twist, warp and lots of other things that you don’t want to happen.

Similarly when conditions are cold and dry the wood fibers contract due to the lack of moisture. This same reaction is what causes your doors to stick in the summer and have big gaps in the winter.

 

How Does Bondo Fail?

So, we’ve established that wood moves but Bondo does not, right? Ultimately, these two products are too dissimilar to stay bonded together for any length of time.

As the wood swells and contracts, swells and contracts over the course of weeks and months the Bondo is gradually shaken loose and pushed out of the wood.

I’ve seen examples of Bondo pushed clear off of a window sill and laying on the ground. It doesn’t take long before at least small cracks open up and water gets trapped in the crevices.

Another thing that can push Bondo out of the wood is called vapor drive. Moisture inside the house will naturally migrate to the outside. This moisture will move through the back side of the wood and come up against the Bondo which is not vapor permeable. It will then exert pressure on the Bondo on its way out of the building.

This same thing happens to paint when there is a moisture problem. You will find paint peeling down to bare wood in these areas. Vapor drive is a powerful force.

You wouldn’t patch wood with cement would you? Most people would think that’s an absurd idea, but auto body filler is much the same when it comes to wood.

I hope that helps you understand and avoid using Bondo to patch wood. Even if your painter thinks it’s a great idea and says he uses it all the time steer clear. It’s only a matter of time before it fails.

If you need to repair wood my post Rotted Wood Repair will show you the proper materials and techniques to get the job done right the first time.

Also check out this short video by fellow restorer Steve Quillian showing a Bondo patch job on some windows after just a few years.

 

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21 thoughts on “Bondo is Never the Answer”

  1. As a restoration contractor with more than 35 years experience I figured I would weigh in on this. I rarely use bondo for repairs with wood. For me it is more a matter of having the right material on hand. Any wood repair that uses a filler non-wood material as a filler needs proper preparation or failure is going to happen. If Bondo or epoxies are put onto a poorly prepared substrate. the Bondo will be more likely to fail. Any repair is only as solid as its substrate. I’m not a huge fan of consolidents, but unless you can get your area being repaired down to clean wood[my preferred method], you have to harden the rotted material. This being done I have found Bondo inferior to Wood-Epox. Interior the failure rate is much lower,but still not as effective as Abitron. My preferred product is Flex-tek by ART[advanced repair technology] Interior or exterior the failure rate is minimal. The system is based on removal of all rotted material and replacing it with new wood using Flextek as glue/filler. Hundreds of repairs on historic properties and no call backs. Mostly lower window sash parts. It is designed to expand and contract just like the wood. Remember any repair depends on good preparation.

  2. this is sensationalist and un-informed. while there are many specific wood repair epoxies out there: when the substrate is consolidated properly first (cleaned with denatured alc., filled with stabilizer, and then bonded to with a low density catalyst, it is not only acceptable but prudent. Albatron, West systems, and other preservation epoxy systems are great for large projects, but they are cost-prohibitive for a DIYer in a rural area without access to specialty hardware stores or special order.

    Bondo is in fact engineered to flex! expansion, contraction, and shock are all a reality of the service life of bondo. It must be mixed and applied properly tho.

    The preservation-grade wood epox versions are great, but anyone with experience in resins, mold making, and form work knows how to gauge a render properly, regardless of the material they are working with.

    Most of the failures of bondo in preservation work have to do with poor understanding of the process, and limitations of filling. Not of the material itself.

    1. EXACTLY! Bondo is like any other quality tool with decades and millions of successful applications behind it, when used correctly it is an excellent choice. It’s used more everyday than any other similar option and the VAST majority of those applications are in fact successful.

        1. There are countless tools and products originally intended for one use yet crossover perfectly well into other industries or for other uses. That doesn’t make doing so incorrect. At all. If the intended results are achieved in an acceptable manner for the user then how they got there IS in fact correct. Bondo on wood succeeds FAR more than it ever fails. It’s one of the most frequently used materials to fill and patch wood daily and has been for decades. It does fail sometimes, there are reasons, there are some other fabulous products like the Abatron line but that doesn’t make Bondo all you’ve claimed it to be. It simply works and has for decades in thousands and thousands of applications. So IT’S NOT never the answer.

    2. Kurtis, one of the problems with Bondo lies in your explanation. Most DIYers don’t know how to gauge a render properly and have zero experience with resins or mold making, but they are encouraged to slap some Bondo on to patch holes by lazy painters or carpenters. This almost always results in failure.
      As a professional I used Bondo for years (following the instructions properly) only to find failure after failure just a few short years down the line. With epoxy systems designed specifically for wood I have not had similar failures. I have also followed other professionals testing that yields similar results as mine.
      In Bondo’s defense they do make wood epoxy versions, but the typical practice of repairing wood structures with auto-body filler is not a realistic practice for long term repair.

      1. Yes, we could say the same about a lot of preservation issues:

        It is Great that heritage group is making fool-proof pre-mixed lime putty mortar for example, but I would rather educate people on how to make their own, and learn the skills to judge their mix and know when it’s right. If we replace understanding of material science through folk learning, then we create a generation of preservation pros with no foundation, just packaged fool-proof solutions.

      2. Scott, you’re absolutely right. I’ve got a few examples of failing Bondo on my own house. These were done by none other than Mr. Kurtis Hord.

      3. I do heritage restoration work and we use Bondo often. The problem I see from the video presented is not Bondo failure, but substrate failure. You can clearly see wood fibres on the back of the peeling Bondo . If the wood had been properly prepared (pressure washed, sanded etc), to ensure a sound substrate, that substrate would have held the Bondo properly.

    3. Always used it in small repairs for wood furniture restoration, never had any issues…..
      BONDO makes their own wood fillet now also

    1. I’ve done exactly that many times. I don’t believe bondo is NEVER the answer. In fact I know this isn’t correct BUT for wood restoration I don’t believe there is a better option than Abatron either. Mixology tints works VERY well with them. I’ve used Transtint successfully with it too.

  3. As true as this is and can be, there are loads of variables including application, mixture ratio, wood species, coatings type, moisture exposure, environmental temperatures, etc. I have seen it fail but I’ve seen it succeed more times than not too. There are better choices but it is most definitely not a certain recipe for failure by ANY stretch of the imagination.

  4. Spot on Scott. Saw an episode of This Old House where Tommy Silva repaired a door jamb with bondo. Very bad shared idea.

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