How Do You Mix Auto Body Filler

How Do You Mix Auto Body Filler?

Out of all the auto body steps to learn, this should be one of the easiest of all. However, this seems to be one of the most confusing. So how do you mix auto body filler? Can it be mixed on the car? Can it be mixed on cardboard? How do you mix it with the hardener?

Mix Auto Body Filler

I have been mixing body fillers for years, over 40 to be exact. I have used many brands of fillers but only one has been the best so far. Where confusion starts for those just starting out in auto body repair, comes from one brand. Everyone always starts with Bondo brand products. It has a popular name and is made by a big company, 3M. Sadly, it is one of the worst fillers on the market and I will tell you why.

How To Judge A Good Filler From A Bad One

For me, curing time is important but it kind of becomes a two-way street. You want a filler that doesn’t take a long time to cure up and become sandable. At the same time, you don’t want it to cure so fast, you don’t have time to apply it correctly. On top of these two factors, you don’t want it to pack up your sandpaper when trying to sand it.

If a filler doesn’t have enough hardener in it, it will fail under a paint job. Too much hardener can also cause failure. However, you are better off it having too much hardener and fulling curing, rather than not. You mix auto body fillers based on temperatures. In warm to hot weather, you mix a little less hardener in it. If it is cold weather, you mix a little more. There is a reason I am bringing up the mixing of hardeners in fillers and the difference between a good and bad filler.

The biggest telltale of a good filler is in the sanding after the filler has cured. A good filler may lightly clog up your sandpaper when you first take the surface off a little. However, it will quickly become easier to sand without clogging. A bad filler such as Bondo and USC. These fillers continually keep clogging the sandpapers. Bondo and USC have to be mixed on the hot side. Meaning too much hardener for them to sand better.

Why Bondo Has Newbies Confused

Mix Auto Body FillerIf you read the instructions on a Bondo brand can or go to their website, you will be told wrong. There is a section on their website that tells you it is best to mix the body filler on the car. This is insane and STUPID, do not ever do that. They also tell you NEVER, mix body fillers on cardboard. Hate to break it to them but ALL real professional body shops do.

Some of the cans tell you, “For every golf ball size of filler, mix 5 to 8 drops of hardener.” The issue to this is, hardener is a cream, it doesn’t come out in drops. If you were mixing fiberglass resin, then yes this would apply. Fiberglass resin mixes with a liquid hardener, so that would make sense. So, for a newbie to auto body repair learning how to mix auto body filler, this is confusing.

Bondo says you can’t mix your filler on cardboard because the solvents that are in the filler soak into it. All I can say is, so what, who cares, you are throwing it away. You are not losing anything by some of it soaking into the cardboard. Their argument is that you are losing some of the hardener. Well, you do not put the hardener onto the cardboard, you put it straight on the filler. To me, it is like Bondo or 3M, never went out in the real world where shops are using fillers. The instructions come off like you are going to mix it and spread it and the job is done. That is not reality.

How Do You Mix Auto Body Fillers Correctly?

If your concern is what to mix it on, here is what you should be looking out for. A mixing board is really nice but isn’t practical. Cardboard is perfectly fine; however, you have to use some common sense. Do not use dirty or contaminated cardboard, it has to be clean. You want to ensure the carboard is plain basic cardboard and not coated with waxes or some other material. Ensure that isn’t a cardboard that flakes or shreds easy as you do not want particles or debris mixed into your filler.

You mix auto body filler as follows. No matter how much you put onto a mixing board, keep the filler in a round circle. From the top of the circle, come down about a quarter way. Squeeze a steady amount of hardener across that circle. Keep the hardener diameter about the size of the hole of the tube it is coming out of. Once you get used to this mixing ratio, you will not always have to ensure that the filler is a perfect circle on the mixing pad.

Bondo’s instructions are confusing because they are trying to give you a reference that you can’t relate. First off, the filler will not go onto a mixing pad the size of a golf ball. It will just keeps spreading out, and secondly, you may need less or way more than that. Bondo also gives you a mixing ratio of 50:1. Who the hell knows how much 50 to 1 is? Mixing creams, isn’t something you place in a measuring cup, you could, but it wouldn’t be practical.

Watch my YouTube video on: How To Mix Body Fillers.

The Hardener

Never just squeeze out the hardener onto the body filler before kneeing it first. No matter if you have just used the hardener, always knee it before every use. There are resins in the hardener that like to separate from the cream part of it.

First take of the cap of the hardener tube. With the tube pointing upward, lightly squeeze the tube till you just get some material wanting to ooze out. This will get the air out of the tube. While the material is ready to come out of the tube, place the cap back on. Now with your fingers, squeeze and massage the tube, mixing the contents inside until it is mixed thoroughly.

Sometimes hardeners will go bad. They do one of a few things. One, you can never get the liquid part to mix with the cream part. Secondly, it can start becoming pasty and doesn’t flow from the tube well. The other thing I usually more to do with the tube itself. If the tube starts feeling brittle, flaky or rubbery, it’s bad. Hardeners can be purchased separately, so never try to use them if they get it any of these conditions.  

The Actual Mixing Of Filler And Hardener

how to mix body fillerWe have talked about the ratio and ensuring the hardener is mixed and ready to use. Now we need to discuss how you actually combine the two. When you mix an auto body filler, you do not stir it like you are whipping a cake batter. Now many, many people show you to mix it with the body filler spreader you are going to spread it with. You can do this and it works, myself, I prefer mixing it with a paint stick. I learned that technique from a true professional from around Detroit.

If you mix it with a paint stick, you get a thorough mix on a board ready to use. That means you have no residual of a hardener or what might not have got mixed on your spreader. Made perfect sense to me, so I have been doing that for the past 40 years with great results.

As from above, you lay your hardener across your filler. No matter is a paint stick or spreader, you pull your hardener into your filler, then your filler into your hardener. This can be done with some speed but not to the point you are whipping it. Fast stir or slapping it around can cause the mixture to react that will cause gasses to start escaping. This will cause pinholes when trying to apply it. You can see how I mixed it in my YouTube Video.

Mixing Boards Or Cardboard?

Mixing boards are nice, they are clean at first and rigid for mixing. They make many mixing boards with disposable sheets and that concept is nice as well. These are perfect if you are going to fix one or two dents and never do body work again. However, if you have a restoration or plan on doing real body work, these board slow you down and become a waste of other materials to keep them usable.

Mixing Board

Once you have your filler mixed, you are on the clock. You don’t have time to do anything but get the filler spread on the area to be worked. Once you are done, you can clean your mixing board.Mix Auto Body Filler Although, usually the filler has started to harden on your mixing board and your spreader. So, you have two things you are in a race to get cleaned before it is too late. Lacquer thinner is what you have to use to clean both items with. Thinners are not cheap anymore, something to consider on job cost.

After a while, there is always leftover hardened filler that gets left on the mixing board. This causes issues of a clean and smooth mix. If the mixing board is the thicker made one, you can’t really flex it to flake off the old filler debris. The thinner boards are easier to clean. They can flex the old material off, however, they become cracked and collect debris in the cracks. This debris starts getting into new mixes, messing up spreading the fillers.

The mixing boards that use disposable sheets are a great concept. However, when you are spreading body filler, you start learning that you have to scrape off the excess on the spreader to apply it correctly. You scrape off the excess on the edges of the mixing board. If you get the filler on the edges of your sheets, you just stuck them together after the filler hardens. There is maintenance and draw backs to using mixing boards. All of which come down to more expense.

Mixing On Cardboard

As mentioned, you have to use the right kind of cardboard. For body shops, they are always receiving body parts in cardboard boxes. These boxes make great mixing boards. You cut them up into 6in by 6in or 12in by 12in squares. In this case, the cardboard is free and disposable.

You can scrape off your excess on the edges without worrying about ruining sheets. You have no cleanup, so you just keep using it up to the filler starting to set. You cut down on cleaning solvents and increase your productivity speed.

Granted, if your cardboard is too thin, this makes it difficult to mix on or scrape up filler to apply. If you get the wrong kind of cardboard that is flaky or deteriorates easily, you can get debris in your filler. The nonsense of worrying about solvents in your filler mix going into the cardboard is just crazy.

Mixing Fillers On The Car

If you want to really know how to mix auto body filler, never ever do this. The idea that a company would write such a stupid method like mixing your filler on the area to be repaired is insane. Think about this. You have an area that you may have welded, that weld created a small crevice. Now imagine, you apply a non-hardened filler over top of it, then apply the hardener. You may never get the activated filler down in that crevice due to the inactive material in the way. This can cause failure in your filler.

There is no reason you would ever mix your filler on top of the area to be repaired. Someone smoking dope wrote those instructions. Definitely someone that has never done professional auto body repair before. Regardless if you are using a mixing board or cardboard. Mix your filler first, then apply it to the damaged area.

My Preferred Auto Body Filler

Mix Auto Body FillerI have been using Evercoat brand fillers for around 40 years, never have I had one issue with it. I started off with Bondo brand products, mostly because that is all most inexperienced people knew. Once I got into going to real automotive paint stores, they carried Evercoat and Upol products. All of the Evercoat products was like magic compared to the crappy Bondo brand.

With Evercoat, I quit using the large amounts of sandpapers. I quit having filler failures, such as cracking or never curing. Even after the Bondo brand cured up, even after a day of sitting. When you run your hand over it, it still felt sticky. Evercoat after 30 minutes felt dry and ready to be sanded.

I could sand the Evercoat with finer papers, which meant less deep of sanding scratches. That meant less spot putties. I found I could get really flat level surfaces because the sanding was easier and more accurate. The Bondo required extra work to get it to start a normal sand, and still packed up the paper. The Evercoat feather edged a million times better than the Bondo. Bondo, even to this day doesn’t bond to the surface as well and chips off around the edges instead of feathering back.

In Conclusion On How To Mix Auto Body Filler

Do not make mixing auto body fillers into rocket science or a science experiment. It does take some practice getting used to how it behaves to the amount of hardener and temperatures. As far as mixing it on cardboard or not, or trying to measure it 50 to 1. All of that nonsense is just that. No professional body shop measures their body fillers. Most body shops use cardboard to mix on as it is more productive.

Everyone has their own styles to doing body work, and in the end, it is the results that count. I have people that argue what paint brand is best, or that dry sanding over wet sanding is better. But in the end, you want a show look, no waves or scratches and for it to last. The guys that achieve this are the ones that perfect their methods and know their tools and materials.

For mixing auto body fillers, just go by the guide from above. Get a clean mixing board, manufactured or cardboard. Place a circle size amount of filler on your board. Run a tube opening size bead of hardener about a quarter way down from the top of your circle, across. Mix with a spreader or paint stick, I prefer the paint stick. Do not stir or whip the mixture fast, this can cause pin holes. Just pull the two parts into each other until the mix is a solid color with no swirls in it.

Applying The Filler

You’re ready to spread your filler. I often lay down a thin tack layer. I push it firmly onto the surface and thin. This is to ensure it has stuck to the dry, clean, grease free surface. I then lay it on close to the level I need it to be. I spread it smooth and always try to ensure that I never leave edges sticking up. I make it thin, feathered out on the edges. Quite often you will find while spreading it, you start to collect it on your spreader. This is where cardboard works better. You can scrape of the excess and use an empty spreader to flow out the filler.

If you have question, feel free to use the comments or contact me through this website.  


  • Dan Jones says:

    Do you recommend a finishing cream as a final step

    • Rodney Parks says:

      If you mean glazing putty as a finishing cream, I do recommend it. It is great for catching small waves, but never use it to fill a heavy scratch, as it will shrink and the scratch will show in the paint job later on. It is great for mild waves and really mild sanding scratches. I always go over my body filler work areas with glazing putty in the end before priming.

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