7/30/2023 0 Comments Bondo all purpose puttyYou 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. You will find paint peeling down to bare wood in these areas. This same thing happens to paint when there is a moisture problem. It will then exert pressure on the Bondo on its way out of the building. This moisture will move through the back side of the wood and come up against the Bondo which is not vapor permeable. Moisture inside the house will naturally migrate to the outside. It doesn’t take long before at least small cracks open up and water gets trapped in the crevices.Īnother thing that can push Bondo out of the wood is called vapor drive. I’ve seen examples of Bondo pushed clear off of a window sill and laying on the ground. 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.Īs 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. This same reaction is what causes your doors to stick in the summer and have big gaps in the winter. Similarly, when conditions are cold and dry, the wood fibers contract due to the lack of moisture. The addition of moisture causes the wood to swell, twist, warp and lots of other things that you don’t want to happen. 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. 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. If that sounds boring, then skip to the next section. If you’re interested in knowing the science behind what happens, I’ll lay it out very simply here. 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. When Bondo (or any auto body filler) is used to patch wood, it fails in a very short time. Just like caulk is too flexible to effectively fill large gaps in wood, Bondo is too stiff to do the same. To fill wood, you need a product that is specifically designed to bend and flex the way wood does. 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. Accordingly, Bondo is a very stiff product when cured, which makes it well designed for auto bodies. Car bodies are very rigid structures that don’t have much, if any, expansion and contraction due to weather conditions. Like I mentioned before, auto body filler is meant to behave much the same way as the metal chassis it is used to repair. If you want to read all about what types of wood fillers I recommend, read my post The 7 Best Products to Patch Wood. These are two very different materials and they require two very different types of filler. That’s what wood filler was designed for. Here’s the deal: auto body filler was designed to fill holes in a steel car chassis not wood. 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. 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.īondo 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! This column was patched with Bondo around the base.
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