There is not a more maligned automotive product than “bondo”. Plastic body filler is most often referred to by the brand name “Bondo” (made by 3M), and its bad reputation comes from decades of poor application and misuse. Nearly every high-end custom car or restoration has some amount body filler on it. Using it on your daily driver or project is not a bad thing, as long as you use it the right way. Learning how to use Bondo the right way isn’t as hard as you think.
History of Bondo
Like most plastic body fillers, Bondo is a lightweight polyester-based plastic body filler that was originally developed as an alternative to lead body solder. Old school body shop techs used to melt strips of lead with a torch to fill body imperfections and joints. The Bondo brand name was created in 1955 by Robert M. Spink, just a few years after the use of lead body solder began to be prohibited. Many bodyshop men of that age were not sold on the idea of using plastic body fillers, but over time, it became apparent that the lightweight, easy to use nature of body filler was a benefit over lead. Not to mention that lead body solder is very dangerous to work with given the need for a torch to heat it, lead fumes, and even lead dust from sanding. In comparison to working with molten lead using plastic body filler just made sense.
Where the bad reputation comes from is misuse and incorrect application. We’ve probably all seen a car in the parking lot that looked like someone slathered on body filler with a trowel. Don’t do that. Plastic body fillers are intended for small dings and smoothing out warped panels, not a major repair itself. It was never intended to fill large dents, rust, or other major body damage. It does not move or or behave like a metal body panel. Why would you waste time laying down a gorgeous paint job if you know the gigantic blob of plastic body filler behind it could actually pop off in the future?
You can actually check whether a car has too much body filler by using a refrigerator magnet. Put a piece of paper between the steel body panel and the magnet (to protect the paint), then see if the magnet sticks to the car. If a magnet won’t stick to a steel body panel, it may be loaded with plastic body filler.
Working With Bondo The Right Way
Now that you know a little more about plastic body filler, you can start working with it the right way. But there are a few key points that you should know before working with body filler:
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”. That does not mean it can’t be done, but the repair will not last as long. Eventually the filler will shrink and possibly crack, or even pop off! The general rule of thumb is that if the dent it deeper than 1/8” you need to do metal work first. Remember, your nice paint job depends on a solid foundation. Simply troweling on the filler may look like the easy button, but that slab of filler may decide to fall off some day (we’ve seen it happen).
Prepare The Foundation
What you are working with needs to be properly prepped BEFORE you apply any body filler. Do not slap Bondo onto a smooth painted surface, it is not going to stick very well. The paint should be sanded off, to the bare metal, and then apply the filler. A 36-grit to 180-grit surface is ideal for body filler application so that the filler has a good rough surface to grip.
Not For Every Location
Don’t use body filler on full gas tanks, or anywhere that gets hot. Bondo is resistant to fuel, but only after it is cured. If you are smoothing out a gas tank, that is great, it you are trying to fix a rusted out gas tank, this is not going to do a good job. Don’t confuse Bondo with JB Weld, the two are not the same.
It Won’t Magically Fix Rust
Body filler is not good for rust repairs. You cannot slather some Bondo over rust and expect it to stop the rust or stick to it for that matter. For rust repairs, you must first remove any rust scale from the metal. If there are holes in the metal, you are much better off cutting it all out, patching it with new metal, and then working the body filler from there. Any rust left under the filler will continue to grow, so don’t be surprised if a small repair turns into a bigger problem down the road. If you absolutely must use body filler to make the repair, then you should remove all the rust scale, add a backup screen to cover the holes from the back side, then use Bondo Bondo-Glass over the rusted area, sand and finish out with regular Bondo body filler for a stronger repair.
Pay Attention To The Mixing Ratio
The mix is crucial. Most of this comes from experience, but you can get close to the optimum mix with this tip- use 1:1 ratio of inch diameter of filler to inch-length of hardener. A 4-inch diameter of raw filler needs a 4-inch ribbon of hardener at room temperature. If it is hotter (above 85 degrees), use slightly less, if is colder (below 65 degrees), use slightly more. If your filler hardens too fast, you used too much, if it takes too long, you didn’t use enough. You only want to mix what you can use in about 10 minutes. Properly mixed, body filler will begin to thicken in 10-15 minutes, and be fully cured in 30. Until you get the hang of mixing it is wise to buy a bit more plastic body filler than your project needs so you have room to try again without running back to the store.
Do not whip the body filler while mixing it. This will incorporate a lot of bubbles in the filler, which means more pinholes to fix later. Use a slow and deliberate figure-X pattern, wiping the filler across itself.
Application Technique Is Key
You can use plastic or metal spreaders, choice is up to your personal preference. Metal spreaders last longer and provide a more defined edge, but plastic spreaders are cheap and can be custom cut to match shapes. Apply the filler to the panel, pushing the filler into any crevices. Wipe the filler as smooth and level as possible, but leave plenty of extra filler above the surface level to sand the repair smooth. Better to have a little too much filler and sand it off, than have to mix up another batch of filler for a low spot.
Get Ready To Sand…A Lot
There are two schools of thought on when to sand filler. If you are doing general bodywork, leveling wavy panels, fixing small dings, etc, then wait until the filler is fully cured, about 20-30 minutes. If you are using a lot of filler or doing some shaping with the filler, you can use 36-grit sandpaper when the filler is in the “green” stage, this is solid, slightly rubbery, but not fully hardened. Using light pressure, you can knock off large amounts of filler to rough-in the shape needed before the filler hardens, making sanding more labor-intensive. And don’t forget the respirator and safety glasses, fine filler dust tend to get everywhere when sanding.
The key to a quality body panel repair is seamless filler work. The edges of the filler should feather out to the existing panel. You should not be able to feel where the metal stops and the filler begins. Any defect you see or feel in the filler will be amplified when the paint is on the body, so check the panel repeatedly. Large repairs require extra effort, as you can easily get waves and ripples in the bodywork when working a large area. Never sand in straight lines, instead use a criss-cross pattern, up and down, side to side. This eliminates waves and results in a much better repair.
Bodywork is a slow, tedious task that takes time to master, but is easy to learn. Don’t be afraid to use Bondo, as it is a good quality product that is a must-have for any body repair project. And don’t worry if you screw up, you can always sand it off and start over.
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A life-long gearhead, Jefferson Bryant spends more time in the shop than anywhere else. His career began in the car audio industry as a shop manager, eventually working his way into a position at Rockford Fosgate as a product designer. In 2003, he began writing tech articles for magazines, and has been working as an automotive journalist ever since. His work has been featured in Car Craft, Hot Rod, Rod & Custom, Truckin’, Mopar Muscle, and many more. Jefferson has also written 4 books and produced countless videos. Jefferson operates Red Dirt Rodz, his personal garage studio, where all of his magazine articles and tech videos are produced.