Car body filler “bondo” is universally used on all cars no matter steal or fiberglass. Bondo is one of the keys to getting that spectacular classic car body look, but it can also be the nightmare that creates long term body problems. In the right hands you will get your dream, but many individuals and restoration shops use it to hide all the real problems never properly corrected.
Bondo, an affordable, quick-fix solution to many auto-repair projects, recently grew to hold a notoriously negative reputation in the restoration industry, primarily due to improper use and faulty application. To clarify, bondo purpose is to act as a bonding agent, a car body filler catalyst that hardens to its surrounding mold and shape with the intention of a lasting durability. Created around 1950, the product functions best when filling small holes and covering insignificant dents but can cause underlying damage when utilized outside of its naturally designed intent.
So, let us start with the basics; ever wonder what Bondo, (the car body filler brand name turned generic — for the purpose of this article, we use bondo and car body filler interchangeably), is made of and how it works? Well, so did we, and that made us do a little investigating into its makeup. It turned out to be harder than we thought to determine the exact chemistry of these materials, but what we found is enough to make some sense of it all.
Car body fillers are materials that use a catalyst to harden. Anyone who has ever mixed up a batch knows that, of course, but few know that the red (sometimes blue) catalyst is MEK peroxide. The “MEK” stands for methyl ethyl ketone, an aromatic chemical compound that is rather dangerous to one’s health, hence the warnings on the filler labels that tell you to use the materials in a ventilated area or use a proper respirator. So much for that part of the mix.
The car body filler itself is a complex mixture of goop that contains, among other things, fiberglass (polyester) resin and talc. The resin allows adhesion as good as most epoxy adhesives and the talc (yes, as in talcum powder) makes the material flow smoothly. Talc is hygroscopic (that is, it absorbs moisture) unfortunately, and that is the reason fillers absorb water. The main solvent in the filler is styrene, which vaporizes as the mixture cures. Bubbles in the filler occur when the mixture cures too fast, trapping the styrene gas.
Car body fillers are thermal-set plastics. That is, they cure with heat (created by the chemical reaction between the filler and the catalyst) and become hard and stable, usually within a few minutes. Too much hardener will cause the cured material to become brittle and crack, while too little delays a cure almost indefinitely.
Temperature is extremely critical to exothermic (the fancy, scientific name for catalyst-induced heat) reactions. In fact, curing is drastically slowed below 64 degrees F ambient air temperature, to the point that it can easily take several days. The best temperature to use filler is between 70-80 degrees F, and that includes the metal you are applying the filler to!
If the metal is cold the mixture will cure from the outside in, trapping moisture against the metal surface. It can also trap unvaporized solvent, which could come back later to haunt you if you paint over it too soon.
Mixing the car body filler on cardboard is not a great idea, since the paper itself will absorb some of the styrene solvent and upset the chemistry. Also, the styrene will release any trapped chemicals in the cardboard, so unless you know precisely where the material came from and how it was handled, use a sheet of glass or plastic or freezer paper.
So How Does Bondo Stick?
Car body fillers stick to the metal through mechanical means. The smoother the metal, the less perfect the adhesion will be, which is why the instructions always tell you to rough sand the metal surface. Those scratches allow the Bondo to stick very well and for an awfully long time, assuming there is no rust layer.
Since we know that fillers stick best to metal, it stands to reason that spreading them over primed metal surfaces can lead to separation later. That is true, but if the surface is still scratched and rough the filler will tend to stick extremely well. If you use an etching primer you must make sure it is properly cured before using filler. Otherwise, the phosphoric acid vapors left over can slow down the cure of the filler resin.
How Thick Can I Apply Bondo?
This is always an issue with body repair and one with no clear answer. There are cars with two inches of filler that go for years with no problems, and then there are cars with a paper-thin layer that lifts off within months. It seems, therefore, that the “secret” of filler’s success or failure is the underlying surface.
The technical data seems to indicate that a 1/8th inch layer is perfectly safe and building up layers to 1/4 inch should not cause failure later. This assumes that the underlying metal is stable, solid, and rust-free. Cracks and holes must be welded because any movement in the metal will crack the filler on top. If your car is of unibody construction, bear in mind that the whole thing twists as it travels along the road. Make sure the metal structure is correct and you should not have any difficulties.
The trick is to use a steel rule to apply filler. That does not mean it cannot be done, but the repair will not last as long. Eventually the filler will shrink and possibly crack or even fall off. The general rule of thumb is that if the dent it deeper than 1/4”, you need to do metal work first.
The Improper Use
If Bondo is a product recently added to your reservoir, consider growing comfortable with the product before applying it to your restoration project. With restoration projects more delicate like classic, vintage, or exotic vehicles, you want to conduct diligent research before applying any betterments to the unit. Depending on the auto body’s condition at the start of restoration, rust might appear as a reoccurring theme throughout the exterior. In this instance, the use of Bondo would potentially harm the remaining structure as it does not treat or prevent rusting, but rather acts as a temporary band-aid to cover the appearance. Through the long haul, the Bondo used on the wrong surfaces or with the intent to cover rust will result in impending complications and future repair invoices. Pay especially close attention to the materials existing on—or materials you intend to apply to—your restoration project and the materials that products such as Bondo are intended for.
Faulty Application
Body fillers like Bondo can ultimately be used to fix specific issues such as mild hail damage and small dents. Carefully planning this process before working with the product and gathering the additional tools needed like sanders, spreaders, and a mixing board will help your application carry on smoothly. However, misunderstanding the application and instruction for applying body filler can lead to a catastrophic result with visible consequences appearing immediately or after a short time lapse. Mishaps such as spreading the product too thick or thin could demise the entire outcome of your restoration project in addition to applying it to too large of a surface. If you feel too uncomfortable to apply the body filler to your restoration project and fear that you may accidentally mis-apply the product, Bondo might not be the best solution for you.
A Penny Saved Now Vs Increased Costs Later
The main objective in restoring a car is to regenerate the auto body back to—or better than—its original form and performance. we understand that large or sentimental restoration projects can grow cumbersome through time and expenses, sometimes causing you to resort to a less expensive option to cover a topical auto body issue. However, if using products such as Bondo improperly, your project will as a result ultimately require additional expenses later. Whenever unsure about a product to use or how its application safely works for your specific restoration project it is best to read and experiment before undertaking a project.
The Problem with Car Dent Repair Fillers Like Bondo
There is no minimum thickness for bondo, but there is absolutely a maximum. You do not want to apply body filler to anything that is thicker than about a 1/4”. For the best applications 1/8″ thickness is where you want to be.
Dent filler has long been a standby for repairing small car dents. By filling in small dents, creases, and holes in the surface of the auto’s body, car body fillers can provide a simple, inexpensive fix for minor cosmetic damage on side panels, bumpers, and more. Even some major dents can be treated with car dental fillers.
Unfortunately, filling compounds such as bondo also present some drawbacks. Fillers like bondo can crack if they are mixed too thickly, harming rather than improving the look of the car. Additionally, there is a risk of discoloration and poor long-term wear when you use a dent repair filler like bondo. If too many side effects arise as the result of using car dent fillers, you risk significantly damaging your vehicle’s resale value in the marketplace.
I have witnessed far too many classic car restorations that were sent to bondo hell with far too much body filler being used on both metal and fiberglass body panels not being properly prepared. The bondo was used to cover up the work not being properly completed underneath.
For those who are having their classic car being restored by a shop then they key is requiring the shop provide plenty of photos of their work at every stage so that you can get a reasonably accurate picture of their work. Especially if you cannot frequently witness the work in person. This holds true for both metal and fiberglass.
If you are buying a classic car or cannot get a lot of photos there are some signs you can look for from the outside such as, wavy body panels, looking for the seams and body panel edges. This will take somewhat of a trained eye to catch the problems.
There are also gauges on the market that will provide a thickness of the paint and filler.
When dealing with metal cars I use a simple and cheap method using a refrigerator magnet; if it sticks, I feel good about the thickness.
So, protect yourself as bondo is an unbelievably cheap way to make classic car body repairs that can come back to haunt you for years if not used correctly.
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