Over the years OEM’s have been making stronger, thinner outer body panels from a variety of substrates.  Outer panels have been constructed Mild Steel or High Strength Steel, Aluminum, Composite material, or some sort of combination of materials and depending on the material used repair procedures differ from easily repairable to unrepairable.  In this article we will talk primarily filler applications on steel panels.

“Bondo” as plastic filler has been referred to over the years, is in actually a brand of body filler from 3M, but it’s also the generic term that makes many body technicians cringe as many think they are sculpting the panel.  Anyone who has ever restored an old car knows many techs start working on the damaged panels only to find themselves salves of the stuff on damaged bodywork.  Many times, they trowel multiple layers on a panel body to “sculpt” it to get a car straight quickly.  Well, it is time to clear up that way of thinking and make some changes.  Body filler is not designed to cover over poor metal straightening techniques.  The panel should be as straight as possible and then the filler should be applied in a thin coat.  Properly used, filler can assist in smoothing out a panel and generally, an application is required when finishing sheet metal in areas that were sectioned and welded.

The first step is to prep the surface to ensure proper adhesion.  Let’s look at the generalized steps:

1. Prior to straightening the vehicle must be washed for any water-soluble contaminates, if the vehicle was not already washed during the damage analysis.

2. The damaged area is now cleaned with wax and grease remover.

3. After straightening the metal with hammer and dolly techniques, you may need to use a few weld-on dent removal pins in areas inaccessible from the backside.  Make sure to check with the OEM to find any foam or adhesive applications in the areas where you will be welding the pins on, as you would not want to degrade or set the material on fire.  Remember most filler manufactures and OEM’s state that filler should not exceed 3mm in thickness.  (Hot tip: use a non-aggressive sanding disc such as, a 3M Green Roloc disc to clean the repair area followed by a dual action sander with P80 grit.  Only use 50 grit on any highly deformed areas.  You do not want to reduce the metal thickness).

4. Feather edge the repaired area using 100, 120, and 150 grit paper.

5. Scuff the area with a Red Scotch-Brite pad.

6. Re-prep the area by cleaning the panel with wax and grease remover.

7. Mask off the panel for application of acid-etch/wash primer on bare metal areas and epoxy primer to the repair area.

8. After the epoxy primer is fully cured, scuff the surface with a Red Scotch-Brite pad, so the filler will adhere properly, by leaving enough scratch in the substrate.

9. After the area had been scuffed, use a vacuum system to remove all dust particles.

10. Before applying any filler, wipe the surface with wax and grease remover and a tack rag to ensure a clean and lint free area.

11. Now it is time to mix up some filler.  Rule number one is that you never mix filler on cardboard.  Two reasons, one cardboard contains a wax like substance to make it somewhat water repellant and two the chemicals in the filler and hardener easily seep into the pores of the cardboard, both causing adhesion and quality issues with the filler.  So either use a pallet made of steel, Plexiglas, or filler mixing paper can be taped to any type of surface.  The idea is that you don’t want it to be porous or contain chemicals.

12. Read the product instructions, as each filler maker has slightly different mixing procedures.  Lay out the amount of filler material you have estimated you need and add the proper ratio of hardener, on the pallet.  Now you will need two plastic applicators (squeegees) or putty knives, one to mix the material and one to apply the material.  Plastic filler material is generally whitish gray or yellowish and the hardener is blue or red in color.  Never stir the material as you create air pockets, which in turn create air holes (pinholes) in the leveled filler.  Flip and chop the material repeatedly until the color is uniform.  Make sure to clean your squeegee or putty knife as you mix, so you do not change the mixing ratio.

13. Using a plastic squeegee, apply the mixed filler to the straightened area.  Apply the material in long linear strokes, medium pressure, avoid adding too much material and in only one direction.  You will also need to follow the contours of the panel.  In some cases, you may use masking tape to prevent material to getting on to bodylines, or adjacent areas.

14. After applying the filler, while waiting for the filler to cure, clean your tools.

15. In cold temperatures a heat lamp may be needed to assist in the curing process. Generally, this can take about 10-15 minutes.

16. Once the filler is cured, you will sand the area.  Generally, you will start with a pneumatic sanded, 8 inch, 6 inch or long board with 80 grit or 100 grit paper, depending on the size of the repaired area.  This will usually be followed by hand sanding boards using 100 grit and 120 grit and finishing with 150 grit.

17. As you sand check the panel for evenness and flatness, periodically with your hand and a non-lint cloth.  This will assist in preventing oils and grease from your hand transferring on to the panel.  As the filler takes shape and becomes smooth, you may want to use a guide coat so you can see the high and low spots while final sanding.  Basically, it is sand and check, sand and check.

18. In some rare instances, you may need to apply another coat of filler to assist in leveling out the panel.  However, if the area is level generally you will need to apply a coat of high quality polyester putty to fill sand scratches and any small imperfections in the filler surface.  If a second coat of filler or putty in needed you should reapply epoxy primer to the area to ensure proper corrosion resistant properties are not compromised.

19. You will mix the putty the same way you mixed the body filler and apply the putty in a similar manner to the filler.

20. When the putty cures apply guidecoat and sand the panel.

21. Hot tip: when sanding filler or putty, always keep an eye on how dull your sandpaper is getting.  When the paper gets dull, it won’t sand any longer and you’ll be working for nothing and in some cases scaring the area.  Always use a fresh piece of paper on the sanding board, especially when doing finishing sanding with finer grit paper.

After final sanding the area with 150 grit paper you will be ready for the feather edge, priming and block sanding process, which is not included and needs to be calculated for each repair area (see our article on the Feather Prime and Block process and calculations in the June 2011 Hammer and Dolly).  When it comes time to apply the high-build filler primer ensure that each coat flashes over properly to avoid shrinkage later.  If the proper flash time is not adhered to, sometime after the vehicle is finished sand scratches and imperfections can become visible and will require re-repairs.

We hope this article has helped the industry to understand the intricacies of applying body filler.
Feel free to contact us at anytime if you have any questions that we could help with.

Larry Montanez, CDA is Co-Owner of P&L Consultants with Peter Pratti Jr. P&L Consultants work with collision repair shops on estimating, production, and proper repair procedures.  P&L conducts repair workshops on MIG & Resistance Welding, Measuring for Estimating, Advanced Estimating Skills.  P&L also conducts investigations for insurers and repair shops for improper repairs, collision reparability, and estimating issues.  P&L can be reached by contacting Larry at Office (718) 891 – 4018; Cell (917) 860 – 3588; Fax (718) 646 – 2733; E-mail larrygoju@aol.com.  Website: www.PnLEstimology.com
Jeff Lange, PE, is president of Lange Technical Services, Ltd. of Deer Park, New York. www.LangeTech.net  Jeff is a Licensed New York State Professional Engineer who specializes in investigating vehicle and component failures. Lange Technical Services, Ltd. is an investigative engineering firm performing forensic vehicle examinations and analysis for accident reconstruction, products liability and insurance issues.   Jeff can be reached at 631-667-6128 or by e-mail at Jeff.Lange@LangeTech.net.