Bondo is Never the Answer

By Scott Sidler • October 26, 2015

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

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.

[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.

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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43 thoughts on “Bondo is Never the Answer”

  1. Repaired bottoms of rotted door frame with bondo about ten years ago. Has been exposed to winter and summer weather every year. Just now, paint is starting to crack in some places. No separation where wood and bondo meet. It’s a great wood filler. Put wax paper over the top of it, and press a door hinge into it for a perfect fit. Wax paper peels right off.

  2. The information out there is very confusing. The information from Bondo says 2-part the body filler is good to use on Wood, metal, ceramics, fiberglass, etc. I looked at a different brand “MinWax”. Their 2-part wood filler says it’s good for wood, metal, ceramics, fiberglass, etc…

    It gives the impression that the main ingredients in the 2-part body filler and the 2-part wood filler are similar. The consistency is the same. They both harden quickly. They both can be sanded and painted. Just to be on the safe side, so far I’ve used the body filler for fiberglass and metal work. I’ve used the wood filler for wood work. So far no problem. My gut tells me they may be interchangeable.

    Just my 2-cents.

    1. Hi Pascal,
      Bondo makes two products that are very similar: Bondo General Purpose Putty and Bondo Body Filler. I believe Bondo Body Filler is the product most people are referring to when they say “Bondo”. The only obvious difference between Bondo General Purpose Putty and Bondo Body Filler is that the general purpose putty comes with a white hardener and the body filler comes with a red filler. I emailed the company and asked what the difference between the two products was. They told me that the only difference was the hardener.

      Bondo also makes a product called Bondo Wood Filler. This product is substantially different than the other two products. While I have had mostly good luck using Bondo Body Filler for wood repair I think the Bondo Wood Filler might be a better product for wood. It costs about twice as much as the other two Bondo products that I mentioned but the extra cost might be worth it. I discussed why in an earlier post.

  3. New Orleans carpenter. I’ve seen it all down here. And yes regular Bondo rarely ever works …EVER… on outdoor repairs. 3M Wood Bondo however works great. Anyone using auto bondo for outdoor wood repair probalbly also uses Alex plus for exterior caulking. LOL…paintable silicone caulk only for my clients…

  4. I am a painter currently working on a heritage restoration. I was obligated to start the exterior portion of the contract in the fall and did extensive patching with Dyna-patch Pro. It held up well over a winter despite going on in cold weather. I think it will be fine. The new carpenters are using auto body filler to finish repairs this spring and swear up and down it will last.
    The company has been around a while so i am reserving judgement but the guys are slobs who constantly borrow gear so my hunch is the use of bondo is a corner cutter for the sake of a quick dry time and i fully expect it to fail. I look forward to charging for fixing it later!

  5. *looks at 10 year old bondo repair on 30 year old Pozzi wood windows*

    I hate to break it to you Scott, but you couldn’t be more wrong when it comes to bondo. For repairs like my wood windows, the trick is to not only thoroughly dry the wood as best as possible, make sure to then apply some sort of wood hardener which gives the bondo much more grip and essentially waterproofs the wood.

    I also used it to repair about 2 inches of wood rot on the bottom edge of my front door. I drilled up into the door, and glued dowels in place in order to give the many layers of bondo something to hold onto. Its lasted over 5 years now. Just re-painted it with some gloss waterbase alkyd/urethane and expect it to be around for many years to come.

    1. I’m not saying you won’t get away with it working, but using something designed to patch sheet metal for wood patching is not something I see working well in my most cases.

      1. You should add a section on metal movement. Or walk a railway in cold months when you have a half inch gap between rail sections and the summer they are butted close.

  6. I use bondo and MDF wood for my art sculptures and they work better for me than anything. In fact I go through many gallons of it. I do paint them but never have I had any problem, they bond perfectly together. So, at least for interior work with MDF and similar types of manufactured wood you definitely can use bondo and dont let anyone tell you otherwise. I’ve got pictures to prove it.

  7. Which Bondo are you being critical of as the 3m marketed Bondo designed for wood is very effective as opposed to using the plastic based auto body filler.

  8. And what if you are going to paint plywood and the plywood has a hole in it? Will using Bondo to fill the hole succeed or fail?

    1. did you read the artical?? bondo is still and doesnt contract/expand. wood does, Bondo will ALWAY fail on wood at some point. get the cheaper (and way better) “wood-filler” option

  9. Scott, What do you think of using Bondo to patch small areas of deteriorated stucco over brink and deteriorated sandstone with a stucco finish?
    Thank you!

  10. Ok, out of my league here but hoping for some advice! I recently commissioned a large 3D A (made from pressure treated plywood, finished with some sort of bar top resin/epoxy and then painted (primer and high gloss) for a high school courtyard. The top of the “feet” of the A are not level and hold significant amounts of water – one foot actually slopes towards the main part of the A and thus holds water near a seam. The artists that did this are nice, but I’m not sure they have enough experience to really say if this will be a problem or not (and they will say it is fine). We will of course need to continually paint this, but standing water plus wood just seems bad (it is doing this in August when it is hot, so I can’t imagine when it is cool).
    So, my question – is some type of filler that levels the surface and slopes it away from the main part of the A a solution for me? I would imagine it would need to be around 1/4 in the thickest part – maybe more – to start the slope. Albatron, Flex-tek, Bondo? To use these products without failing, would we need to sand all the way through the paint and sealer levels to get to wood or would a serious scuffing/sanding suffice? Thanks for any help anyone can offer!

    1. The feet of your “A” have a fundamental design flaw and the only proper way to fix them is to rebuild them. Anything else is a bandaid, which actually may make things worse. Standing water on wood as well as many other materials is never good. Proper drainage and prevention from absorption are key in the design of any structure that will be exposed to the elements. The “artists” need to learn from this so they take drainage into consideration in future projects, and should be tasked with fixing the structure. This means repositioning the horizontal faces so they drain water off to the ground. And, further the ground should slope away from the structure. It is not acceptable to have an exterior horizontal surface that will create a basin, especially over a seam. If even the tiniest bit of water seeps in and freezes, you’ll have a bigger seam. If it doesn’t freeze, it will seep into the edges of the plywood, causing the wood to bloat. Each cycle of seeping, drying and seeping will make it worse.

  11. I’ve used Bondo for wood repair for twenty years or so. I’ve used it to repair wood windows and rafter tails mostly. The results have been good but there have been problems:
    1. Bondo shrinks. This isn’t a huge problem but it does require a patch to be overfilled and sanded down.
    2. When Bondo is feathered the repair can fail over time.
    3. Bondo is a bit difficult to apply. It sticks to application device and as it is spread it is dragged away from where you want it to stay.

    I recently tried the Bondo Wood Filler. I don’t have experience with it over time but it seems like it might be a significant improvement. It had no noticeable shrinkage, it was much easier to apply and it seemed to adhere better to the wood. I suspect that it isn’t as hard or strong as normal Bondo but I don’t think that is an issue for my purposes. I looked for some of the products mentioned in this blog, but I didn’t find them at Lowes or Home Depot. I found Bondo Wood Filler at Home Depot. It is substantially more expensive than normal Bondo but the cost is still small if it is used for a small amount of routine patching. I paint it so the fact that it is wood colored isn’t a feature for my purposes but it seemed like it might take stain and be a fairly close match to wood if is used to patch wood that will be stained and clear coated.

    Bondo All Purpose Putty: I believe this product is the same as standard Bondo except that it comes with a white hardener instead of red. It might be marginally better for a purpose where a gray patching compound instead of a red patching compound is desired but it is harder to see when the white hardener has been mixed in thoroughly.

  12. Help. I am fixing wood rot myself and have never done this before. I did watch a bunch of YouTube videos and some where in it all got very confused. I saw that Abatron was rated number one but no one had it in stock by where I live so some how I got on Bondo. Some where in the process I ended up getting Bondo all purpose putty. I have significant wood rot and scraped it all out with huge holes and huge part of the frame on my front door gone I ended up using Bondo all purpose putty to fill in and have filled it in about 15 times to the point now it’s molded and sculptured and I am almost done. After reading about your comments on Bondo now I am very worried. I am very concerned that this is going to fail and not look good within a couple of years. What would you suggest I do from here? This is some significant bondo layered on top of each other to make it look like wood. I am a simple girl just trying to fix my front door. I don’t have any electric powered tools to scrape it back off and not sure how I would even get it off because it’s like cement now. Help? Can you give me some suggestions?

  13. I have used body filler for filling voids on some exterior and interior wood projects. So far, no failures. I also use CA and ground turquoise on very small voids in some wood furniture products. CA and turquoise were not designed for filling voids, and may not be the “correct” use, but it sure is purty.

  14. I’d like to see an example of Bondo failing. No pictures? And trying to fill a 3/4″ gap doesn’t count. Also, there ARE different Bondos for different projects. I do not think you have yet grasped how to use it…

  15. 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.

  16. 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.

  17. 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.

  18. 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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