How to Mix Bondo

Reviewed by Jason Shackelford

Last Updated: July 24, 2024 Fact Checked

If your automobile is rusty or you get in a collision and the damage isn't too extensive, you can patch the dings with a body repair product called Bondo. Learn to mix Bondo and save money, as Bondo, also called body putty, is considerably less expensive than a body shop. Applied correctly, Bondo repairs are durable and long-lasting.

Combining Bondo Filler & Hardener

Scoop a golf ball-sized dollop of filler onto a flat surface and flatten it into a circle. Knead the tube of hardener, then squeeze one line of hardener across the circle. Fold and scrape with a flat spreader to combine until it forms an even, salmon color. Avoid circular motions, which introduce air bubbles.

Steps

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Alternatively, you can mix it on particle board or buy an inexpensive pad of disposable plastic sheets to mix the Bondo.^{[1]
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Open the can and mix the Bondo with a stir stick if there is a layer of liquid on top. Use the stir stick to lift a golf ball-sized dollop of filler out of the can and reseal the can.^{[2]
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- Mix a larger amount of Bondo and hardener if the area you need to patch is larger. Look at your project to estimate the amount of Bondo needed.
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Add 1 line of hardener--also known as a catalyst or fixing agent--by squeezing it out of the tube across the top of the Bondo filler. (Bondo comes with a tube of hardener.)^{[3]
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- If you add too much hardener, the mixture will gel too quickly. If you add too little, it increases the amount of time for it to cure, and if you don't add enough it will never cure. If you add 1 line of hardener across the top of the dollop of Bondo, it takes about 15 minutes to dry after applying. Add 2 lines and it takes about 5 minutes to dry. Experiment to get the right Bondo mixture.
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Use an ice cream novelty stick or a plastic spreader to quickly mix the two substances together. Do not whip them, but rather fold them to avoid introducing air bubbles.
- Use hardener with dye in it so you can see when the mixture is mixed evenly. If there are no streaks in the filler or areas that are darker or lighter, it is mixed well.

Community Q&A

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Question

What's the right ratio of hardener to Bondo? For example, how much hardener should I mix in with two ounces of Bondo?

Community Answer

Mixing Bondo with the hardener is almost an art form. Start with two ounces of Bondo, squeeze out a few corn-kernel-sized drops of hardener, which is typically red in color, and begin to mix it into the Bondo. When the mixture turns a light red in color, you've got a good ratio.

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At what temperature can I use Bondo?

Community Answer

I wouldn't suggest using Bondo in temperatures below 60-65 degrees. If it's a sunny day and you can't work in the shade, then avoid temperatures higher than 85 degrees in the sun.

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Question

Why do you recommend mixing bondo on something that will soak up resins and solvents in the bondo and the catalyst?

Uglymug18

Community Answer

It is not recommended. You will want to mix on a smooth almost pore free surface. You do not want to mix on something that will soak up any of the material.

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Tips

Buy plastic body filler at hardware stores, auto-parts stores or discount department stores.

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If you want to use Bondo Glass, the filler and hardener are mixed in the same proportions as regular Bondo, and the mixture is folded in the same way till there are no streaks of red hardener in the gray Bondo Glass.

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Once the Bondo and hardener are mixed, you have only about 3 minutes to apply it before it starts to gel. After it gels, it won't stick to the surface you are repairing.

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Warnings

Don't use metal tools like screwdrivers to mix Bondo because doing so may add grease or oil to the Bondo mixture.

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Always wear eye protection.^{[4]
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Mix Bondo outside where there is plenty of fresh air circulating, as the fumes may be dangerous.^{[5]
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