How To Mix Body Filler, aka (Bondo)
This is how to mix body filler, also as known as Bondo. First, you need a Bondo board.
Get started with a piece of heavy gauge sheet metal or (what my father used and taught me to use) a piece of glass that’s glued to a wooden board of some kind to mix on. This way it won’t crack or chip if you bang it up a little. So go ahead and make yourself a body filler mixing board that you can use for years to come.
This is recommended for a fast clean up and you’ll end up using that board for years to come, trust me. A nice razor blade cleaning when you’re done and your Bondo board will be good as new every-time.
If you’re a beginner I recommend by starting with small mixing amounts of filler so it doesn’t harden before you’re finished shaping it on the panel. You can always mix more if needed.
Important Reminders: After you open up your can of body filler can for the first time, mix it. See how I do this in the video below. Also, don’t forget to mix your hardener while it’s in the tube.
How Much Hardener Do You Add To Your Body Filler?
You need to be sure to add the right amount of hardener to your Bondo; adding more than enough will cause your body filler to dry faster and give you less time for shaping and laying your filler on your panels.
By adding not enough hardener; you’ll end up with a pale looking filler color that will take hours to dry and sometimes it won’t even dry at all. You’ll end up scraping it all off just to do it again.
Like my Father always said “for every golf ball size of body filler, put about 5-8 drops of hardener in it”. Go ahead. Test that out! Once you get the technique down, you’ll be able to add in lines of hardener like I do in my videos.
Make sure after mixing that your putty looks uniform and one color. Usually body filler is a light grey color and hardeners are blue or red. In the end you should have a putty that looks pinkish or a light bluish color.
If you don’t mix your putty thoroughly you’ll end up with soft spots in your filler and you don’t want that. The chemical reaction between body filler and the hardener also depends greatly on room temperature It will dry much faster in a warm or hot day rather than a colder day so keep that in mind.
Body Filler Spreaders and Applicators
You have the option of using many different sizes of applicators. I used to use the metal ones when I first started because it’s what my father used most if the time. But now, I like the plastic ones. Why? Simple. Because it’s easy to clean up.
After the putty hardens on your applicator you can just bend your plastic applicator and peal the Bondo right off. They come in many sizes like 3″ 6″ and you can easily cut them smaller if you need a certain size.
Body Filler/ Bondo FAQ
Q. Can I apply body filler over a painted surface with out sanding?
A. No. You should sand to the bare surface in most cases. If filling over shallow dents or over fiberglass, then make sure that the surface is sanded with at least a 100 grit paper so the filler has a surface to adhere to.
Q. Is body filler waterproof or resistant?
A. Yes. Tests show that body filler only absorbs 0.3% which is enough to say yes, it is water resistant. You can apply your filler, get it wet and it will be fine. Applying primer and paint will make it waterproof.
Q. What type of paint can be applied over Bondo?
A. All kinds of automotive paints, rustoleum paint, even house based latex and oil based paints
Q. When can body filler be painted?
A. 45-60 minutes after it’s cured and basically prepped for paint. Say, you laid your filler over a small area and let it site for 10 minutes to cure. Takes you 10 minutes to sand it to shape, 35 minutes for primer and cure time, another 10 minutes to wet sand and have ready for the painting process.
Q. How thick should fillers be applied?
A. Generally you want to stay as thin as possible and under 1/4″ thick. But I’ve been in this business way too long to say that’s what everybody does (body shops included) I’ve seen and ground off 1 inch thick of Bondo off of cars. I like to say, stay thin as possible all over. Try not to fill for correct gapping or near door, hood and trunk edges and corners.
Q. My body filler is not curing why?
A. Four main reasons. 1. Not enough hardener mixed with your filler. 2. The cream hardener is old and passed it’s usual 1-1/2 year shelf life. 3. Not mixed properly (Bondo still grayish in color or same as original color) 4. May have been mixed in a colder than recommended conditions.
Q. What is the mixing ratio for body filler and cream hardener?
A. Use a golf ball size amount of filler and use 5-8 drops of hardener or about and inch and a half strip of hardener.
Q. When can body filler be sanded?
A. 20-30 minutes after applied. Or a few days later.
Q. What is causing my body filler to have pinholes?
A. Using too much hardener can cause gassing, mixing in a circular motion which increases and forces air into the mix. Mixing should be done in a back and fourth wiping and folding motion.
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