How to use Bondo Body Filler on Wood?

Perhaps the most useful substance in the DIYers arsenal, Bondo can be used for everything from repairing rotted window sills to patching drywall to restoring old furniture.

The two-part resin was originally formulated for auto body repair – but its unique characteristics also make it ideal for repairing damaged wood.

Can Bondo be used on wood?

Bondo body filler has several features that make it ideal for wood repair.

It dries quickly, making timely repairs and multiple coats take far less time than other repair methods. It also bonds to itself, which means you can layer multiple coats on top of one another to achieve a particular shape. Bondo will not shrink – which results in a rock-solid repair that will last for longer than you’d expect.

Last but not least, Bondo is pretty cheap. A can of Body Body Filler typically costs less than half the amount of other wood fillers and will yield similar results to those pricier products.

How to use Bondo Body Filler on wood – Guide

If you’ve never used Bondo before, there are a few things you’ll want to keep in mind to get a good result. I’ll break down what you need to know in step-by-step detail.

Tools & Materials Needed

Step 1: Clean and Remove Old Material

The first step in a successful application is to remove any weak, rotten, or loose material from the wood. In my case, I’ll be using the Bondo to patch old, rotted wodden window trim.

You can use a hammer and chisel, a pair of needle-nose pliers, or a small pry bar to loosen and remove any weakened material. Test if the remaining wood is sound by pushing the end of a screwdriver into it, if it crumbles, then it’s got to go. Clean the gap of any dust or wood chips.

using a chisel to remove rotted wood
A hammer and chisel work well for removing soft, rotted wood.

After the weakened material is gone, make sure the remaining wood is thoroughly dried before applying Bondo. Moisture doesn’t play well with Bondo, so you may need to use a hairdryer to remove any dampness.

Step 2 (Optional): Apply Wood Hardener

Wood harder is a liquid product that penetrates deep into wood fibers and binds them. It essentially petrifies wood and will turn old rotten wood into rock-hard material that repels moisture.

You don’t need to use a wood hardener for every situation, but it’s particularly useful when you’re making repairs in areas that are subject to water damage. Windows, doorways, decks, and other exterior surfaces can all benefit from wood hardener.

Apply the wood harder as per the directions, and let it dry completely before continuing.

Step 3: Prepare the Surface for Bondo

If you’re making a larger repair, one way to ensure the Bondo will adhere well is to add some form of anchor. Many carpenters will drive several screws into the material which will act as an anchor for the Bondo, giving it more surface area to grip onto.

For larger gaps in the wood, you can cut a piece of wood to fill the majority of the gap, and then fill the remainder with Bondo. This will save you from having to use multiple coats and will reduce the amount of Bondo you need.

Step 4: Mix up a Batch

Now it’s time to mix the two parts of the resin together. Bondo comes in several different types, with some having red-colored hardener and some white. I prefer the red, as it makes it easy to see when you’ve mixed the harder into the putty sufficiently. It will turn a pinkish-red color when mixed together.

Before you begin, make sure you’re working in a well-ventilated area, and have plastic gloves/safety mask. Bondo is strong stuff, and you don’t want to be breathing it in if you can help it.

bondo putty and cream hardener
Bondo putty with cream hardener before mixing.

The rule of thumb for Bondo is 1-inch of hardener per golfball-sized section of putty. If you add too much hardener it will become unworkable too quickly, and not enough will result in putty that doesn’t dry correctly.

My advice is to mix small batches even if it means you’ll need to apply several coats. After you mix in the hardener, you’ve only got about 10 -15 minutes to work with the putty. After that point, it becomes too gummy to shape easily.

bondo mixed together
You’ll know the Bondo is mixed together properly when it turns a uniform pinkish-red color.

Also, keep in mind that heat has an effect on the hardening speed. Bondo can harden in less than 5 minutes in hot summer weather, so you may want to apply it when the temperature is a bit cooler.

Step 5: Apply Bondo to Surface

Use a putty knife to scoop and apply the mixed resin into the gap. As you work the Bondo into place, try to achieve a shape as close to your intended finished look as possible. Once it hardens completely it will be much more difficult to sand away.

If the area you’re applying the Bondo to is wide or deep, consider using several coats.

Step 6: Sand Away Excess

About 30 minutes after you mix up a batch, the resin should be hard enough to sand. As mentioned in the last step, try to remove as much excess material as possible prior to sanding – this stuff dries rock-solid.

sanding away excess bondo
Sanding away excess Bondo.

Sand away until you’ve achieved a nice flush finish. A power sander will make this task much easier. Keep a mask on while you’re sanding – you don’t want to breathe the dust in.

Step 7: Paint and Primer

Now, all that’s left to do is apply a few coats of primer/paint to your finished product. Keep in mind Bondo Body Filler doesn’t take wood stain very well. It will stick out like a sore thumb unless you paint it over.

Bondo also makes a special wood filler product, which is designed to take wood stain. If you’re planning on staining your finished product, you’ll want to use Bondo type.

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