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 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.