How to Use Body Filler - Show Car Tips from Start to Finish
We’ve been busy preparing our 1968 C10 project truck for paint. Even though we used new, reproduction truck body panels throughout, additional shaping and filling is necessary for the show-stopping result that we’re after. That means it’s time for body filler. In the video below, our friend Ben at No Coast Custom and Rod Shop gives us a 101-level intro on how to use body filler and body filler techniques. And in the text below, we’re going to take a deep look at what filler is and provide some body filler tips.
What is Body Filler?
Even the very best metal shapers in the world can’t shape a panel that can go straight into the paint booth for final finish. Some sort of putty or substance that is easy to sculpt and sand into a perfectly smooth surface has been necessary for about as long as we’ve been putting shiny paint on our cars. In the early days, this substance was lead body solder. Panels were heated with a torch and sticks of lead were melted to the surface and shaped with wooden paddles before the cooled blob was filed and sanded. Ever wonder where the term “lead sled” came from? Early customizers used lead to achieve those famous smooth, molded profiles of those early Mercs and Fords.
In the mid-fifties, another product became available that was far safer and easier to use than lead. Plastic filler, or “Bondo” was introduced and almost overnight became the auto body industry standard. Body filler is a two-part mix of polyester resin with a separate hardener. When combined, a chemical reaction causes the creamy filler to begin to set up into a hard compound that is easy to sand and sculpt. It is applied while still in paste form, then left to cure prior to sanding.
Body Filler vs. Bondo
Stay with us here; all Bondo is body filler but not all body filler is Bondo. Got it? Similar to Kleenex and Ping Pong, Bondo is a trade name for the first widely available plastic filler and is often used interchangeably with all types of plastic filler. So we often say that our project is “ready for Bondo” even if the filler we’re using is from a different manufacturer.
Is Bondo Bad?
Body filler has developed something of a bad reputation over the years thanks to amateurs asking it to do things that it was not designed to do. We often deride cars with bad bodywork as being “full of Bondo.” For further reference, see that rusty 80’s pickup in the backlot at your local used car dealer with chunks of poorly applied body filler ready to fall off of it. But when used properly, body filler is necessary to achieve professional results. That show-stopping hot rod on the cover of all the magazines has plenty of plastic filler in it. And those gleaming bare-metal features that you see from time to time? Even those cars will get surfaced with plastic filler in preparation for paint. Bondo is not bad when used properly.
How Long Does Bondo Last?
When properly applied to a car that is well cared for, body filler will essentially last forever. However, we’ve all seen plenty of examples of body filler failures that at a glance, seem to be the fault of the product. A closer look will often reveal that other factors are to blame.
Poor preparation or application of body filler is a common cause of these failures. Body filler must be applied in thin coats to clean surfaces. If the base metal is not properly prepared, the filler will not adhere properly. Similarly, filler is not designed to fill holes. While we’ve all seen epic fails when filler is used to cover up big nasty rust holes, even small trim holes must be filled with metal or a more substantial product before filler is applied.
Another cause of failure is environmental exposure. Body filler will not fare well if exposed to sun or moisture. If your project has stalled out in the body work phase and the bare filler is repeatedly exposed to sun or moisture, the filler will degrade. Same for the finished car that is left to sit outside. Moisture has a way under the paint that seals the surface, and repeated exposure will cause the filler to fail.
How to use Body Filler/Bondo
To avoid those pitfalls that have given body filler a bad rap, it’s important to understand when and how to use it. Filler is designed to be applied to a clean, properly prepared (more on this later) surface in thin layers. This part is important; body filler is not going to magically fix that bashed in quarter panel or inch-deep hail dent. Always get the metal as straight as possible before moving on to filler. Extra time spent with a hammer, dolly, or shrinking disc will set you up for success when it comes time to apply filler and finish the panel.
Body Filler over Primer, Paint, or Bare Metal
There are many filler products on the market and each manufacturer will have their own recommendations when it comes to surface prep. Similarly, some products are designed to be better suited to certain applications than others. For example, on our C10 project, we’re using a fiberglass reinforced body filler over bare metal weld seams, generally saving the regular polyester filler that you see us using in the video for surfaces that have already been prepped and primed with epoxy. With all that said, there are some general rules of thumb that apply to many types of body filler.
First, rarely if ever is it recommended to apply filler over paint. Automotive paint is a hard shell that is meant to repel things from its surface. That means that products like filler and primer will not adhere well to painted surfaces. Blast, strip, grind, or sand first before moving on to bodywork.
If you’re applying filler over bare metal, it’s important that the surface has some tooth for the filler to bite. Once the panel has been straightened and prepped with something like 80-grit on a DA sander, then thoroughly cleaned with wax and grease remover, a 36-grit roloc disc will do a great job of roughing up the surface. Take care to visually inspect for any pinholes that may be hiding rust or moisture, as these may wait to become problem areas until after that expensive paint has been sprayed. Also be sure to wipe the panel with wax and grease remover before the 36-grit step, as that tooth may catch lint from the wiping cloth and contaminate the surface.
Generally, bare metal will be prepped with epoxy primer to protect from corrosion while work is taking place. Epoxy also makes a great foundation for filler as long as it is properly prepared and the epoxy is fully cured. Start by sanding the epoxy with 220-grit on a block. This will not only rough up the surface, but will also show highs and lows to identify where filler will be needed. You may find it helpful to mark the low spots with a pencil to help guide your filler application. Those low areas will still be shiny, so they will need to be hand-sanded, first with 220, then a red Scoth-Brite pad. The shine should disappear, leaving only those pencil marks as a guide. Wipe once more with wax and grease remover and you’re ready for filler.
It can take some practice to perfect the application and sanding of body filler. But ultimately, understanding the best practices described here and following the instructions that accompany whatever body filler you are using will set you up for a successful, long lasting finish on your project.