Watch this real-time video of me mixing and applying body filler on the 1997 Mazda Miata project at my home shop.
Please forgive the audio, I had my mask on. The quality and audio within the VIP course is much better. Plus you get full-step-by-step process that you can directly copy and apply to your projects, not to mention the hundreds of dollars worth of bonuses that you get as a VIP insider.
Below is another video on the body filler mixing process in it’s simplest form.
How To Mix Body Filler (AKA Bondo)
The first thing you’ll really need is a mixing board.
Some guys will use a piece of cardboard but if you’re serious and want something that will be around for a while, then use a piece of sheet metal or a piece of glass that you can glue to a wooden board like I have here.
The reason why I like glass is that you can easily scrape it clean with a razor after each use. It will be ready when it’s time to mix your next batch of body filler.
Mixing Body Filler
If you’re a beginner at mixing body filler, Bondo, I recommend that you start in small golf ball size amounts.
For every golf ball size of filler you’ll want to add about 5-8 drops of your hardener.
When you first open a can of body filler you may see a film of glaze above your filler, if you do, just mix it up by using a paint stick (like in my video above). After mixing and having an even consistency, then you’re good to go.
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 or pinholes and you don’t want that. The chemical reaction between body filler and the hardener also depends greatly on room temperature of you workshop. I would say anywhere from 70 degrees to 95 degrees. If it’s in that 90’s range in your workspace a shop fan always helps.
Make sure after mixing your putty that it looks uniform and is one color. And remember, that it will dry much faster on a warm or hot day rather than a colder day. Give yourself time and touch it every few minutes to check how it’s setting up. Once you are at a tack free stage, and the filler is starting to feel hot on the panel (because of the chemical reaction and it setting up) you will be ready for the rasping stage if you decide to use a rasp to shape down your filler. If not, let it setup and dry hard, and you can proceed to sanding your filler down with your 80 grit sand paper. Using the correct sandpapers is a very important process when doing body work.
The more filler you mix and apply, the more you’ll get the hang of it. You’ll also start to notice if you’ve added to much or have too little hardener in your body filler by the color.
Body Filler Spreaders and Applicators
You have the option of using many different sizes of applicators. In the past, I’ve used the metal types because it’s what my father used most of the time. But now, I like the plastic ones. Why? Simple. Because their easy to clean up and flex when you need them to.
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. The larger metal 6″ style putty knives are great for larger work where you are adding a skin coat of Bondo across an entire fender, hood or door.
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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