Plastic body filler – sometimes called putty, mud, or by its original brand name Bondo – has been the filler choice since the 1960s. It's commonly used when repairing vehicles after dents have been removed, high spots reduced and imperfections eliminated. What remains is a straight but uneven surface that must be smoothed out.
Many companies manufacture body filler under several brand names, but in general it is a two-part catalyzed body filler, the larger part being the polyester rosins and the smaller tube being the catalyst (organic peroxide. Combining the two starts a chemical reaction to harden the substance. After application and hardening, it can be sanded to the proper contour, to provide an undetectable repair. Plastic body filler was designed as a convenient and less expensive alternative to body solder, which was used in the early days of collision repair. Body solder is more expensive and more difficult to apply, but its main drawback is high toxicity. Lead-based body solder posed toxic hazards for people and the environment.
Plastic body fillers are not without their own hazards, and technicians should read and follow the personal safety recommendations listed in the MSDS. Plastic body fillers' fumes can be quite toxic in high concentrations, and the hardener can cause burns with prolonged skin contact. Safety glasses, gloves and respirators all are recommended when mixing and applying body filler. During sanding, technicians should use proper personal protection equipment to avoid inhaling particulate matter (dust).
Although plastic body fillers were originally designed to be used to repair vehicles, they also have been adapted for use in woodworking and other areas of repair where fillers are needed. In fact, at the 2010 Daytona 500, officials patched a pothole in the track using plastic body filler, which held up for the remaining 40 laps before it was permanently repaired.
Plastic body fillers come in different types. Fiberglass fillers are designed to repair large dents, rusted-out areas and tears or cracks in metal, fiberglass or wood. Their manufacturers claim this is a premium filler that is rustproof and waterproof. These fillers come with standard polyester rosins and fillers, but have strands of fiberglass reinforcement added for durability and strength. When the hardener is added and mixed thoroughly, the substance has a short period of time when it remains pliable and can be spread in the area of repair. Following the cure time, the substance hardens and can be sanded to the proper contour for a durable repair.
Other products are marketed as light body filler, which comes without fiberglass and is more commonly used to repair the dented areas that have been metal finished and brought back to their original shape and state. Then a small layer of plastic body filler is used to smooth out the surface.
Polyester glazing body fillers are for those times when a light, smooth and durable coat is needed for finish work. Some body technicians prefer to use glazing compound as an additive to light body filler, which thins it out and allows the technician to apply a smoother and more precise final coat.
Surface preparation
Before plastic body filler can be applied following the dent repair, paint must be removed from the dented area using what is referred to as the "buff grind" method (Fig 1). This is accomplished by holding the grinder as flat as possible, which quickly removes the paint from the high surface, revealing the low surface. Paint that remains in the lower areas can be removed by holding the grinder at a higher angle. As shown in Figure 2, the lower areas, which were ground at a higher angle, reveal a different surface texture and appearance. Those areas can now be raised using a weld-on pulling tool such as seen in Figure 3, and a pick hammer; or a hammer and dolly method can be used, if access to the underside is available.
Although some body fillers are marketed as direct-to-metal or corrosion-resistant, most recommendations by paint manufacturers suggest the application of a corrosion-resistant primer, such as acid etch or epoxy primer, to restore the corrosion resistance removed by grinding. Also remember that if the hammer and dolly method was used, the areas of impact on the underside of the panel have removed the corrosion protection materials due to the hammer impact, resulting in what are known as corrosion hot spots. Treat all areas of potential corrosion before applying body filler. A corrosion resistant primer can be applied using a foam brush or roller (Fig. 4) rather than spraying. The surface needed for application of body filler does not require the same smoothness as paint; therefore, application with a roller, which is much quicker (Fig 5) than the spray method, will be effective.
The body area that has had the paint removed around the dent should be feathered with a DA to create a smooth transition from the bare metal to the painted surface. Apply corrosion-resistant primer, covering bare metal prior to the application of body filler. By using a roller or brush, the corrosion-resistant primer can easily be reapplied after sanding the body filler if any breakthroughs occur.
Mixing
Plastic body filler, as the can lid indicates (Fig 6), should be mixed using a vertical-stir method (Fig 7). That is, the paint stick is pushed down from the top, continuing to the bottom, scraping forward to the center of the can, and raised out. Then the can is turned slightly and the procedure is repeated until the entire can is thoroughly mixed. Body filler can dry out quickly on the bottom of the can, or when the can lid has not been replaced between uses, as seen in Figure 8. If this happens, products are available that can be added to remoisten the filler.
Most body fillers are catalyzed at a 10-1 ratio, which means the body filler rosins are (by volume) 10 times more than the hardener. As seen in Figure 9, a 4-inch wide pool of body filler is applied to a nonporous mixing surface and a full line of hardener is applied. Never use cardboard because it soaks up the rosins and the catalyst, creating a poor mixture. The body filler is then mixed until all the streaks of color (hardener) are removed (Fig 10). Notice that the filler is being folded and pulled with downward pressure, kneading out the air bubbles (Fig 11). Never stir (Fig. 12), which increases the amount of air whipped into the filler, causing pinholes that will need to be filled later.
Over-catalyzed filler (adding too much hardener) also will cause an excess amount of pinholes because of the heat generated by the catalyst. A thorough application of body filler can be achieved in one application. If the method outlined below is practiced, the technician can achieve body filler application with a single coat. This will increase productivity.
Some technicians mix standard light body filler with polyester glazing putty, usually in a ratio of three parts body filler to one part glazing putty (Fig 13). This thins out the body filler, and some techs feel that a smoother application of body filler can be accomplished. Also, the addition of glazing compound to slightly dried-out body filler can bring it back to its original smoothness, aiding in the application of the filler.
Body filler separates easily during storage and should be thoroughly kneaded with your fingers (Fig 14) before using. If the tube is not kneaded properly, a thin, watery, ineffective hardener will come out. Body filler and hardener come in different colors (Fig 15) such as red, blue and white. In the past, the only difference between these colors was the color. Now some manufacturers offer hardeners in three different hardening speeds for cold, medium and hot weather. As ambient temperature rises, the catalyzing process is speeded up. So often, in very hot weather, when plastic body filler is mixed, it will harden so rapidly that a smooth application is impossible. An alternative is to add the hardener beside the filler, walk to the vehicle, mix and apply, usually in small batches.
Application
After the surface has been properly prepared and the body filler mixed, application is critical. First, a very thin coat of body filler is spread over the entire prepared area (Fig 16). This wets the surface so a thicker coat can be immediately applied over that wetted surface (Fig 17). After several thinner coats have been applied to the area to be filled and the surface of the body filler is as thick as necessary (never to exceed 1/4 inch, with 1/8 inch preferred), the edges are feathered (Fig 18) to facilitate sanding. Notice in Figure 18 that the lower edges are smoothly feathered, and a second pass will be drawn across the top to feather it as well. The smoother the application of body filler, the quicker the sanding will be completed.
Sanding/smoothing
Sanding or smoothing the surface of the body filler is accomplished by using a block and sandpaper, usually starting with P80 grit sandpaper. When body filler is applied and hardening takes place, the surface may still feel a bit oily to the touch (Fig 19); and in fact this sticky surface will quickly load your sandpaper. Often body techs will keep a piece of used P80 grit paper on hand to be used with this first sanding application, instead of sacrificing a new piece. After the initial sanding to remove the sticky surface, apply a guide coat to reveal high and low spots. Although a guide coat (Fig 20) is most often used when block sanding primer, it also is an excellent help when sanding plastic body filler. A vacuum sander helps remove the debris, which can be significant when sanding plastic body filler.
After the body filler has been sanded smooth and flat, use a DA with P80 grit sandpaper to feather the surface. A guide coat is applied prior to using a DA sander, which helps reveal high and low spots and indicates when to stop sanding. With the help of guide coats and continued sanding to the appropriate grit for priming (some companies recommend P220 or P320), this process is designed to remove any linear scratches produced by the block sanding or swirl scratches produced by the coarse DA sanding.
When it seems that the surface is completely prepared for priming, the technician should feel the surface to make sure no imperfections remain. To help with this process, a piece of cloth or paper can be placed between the surface and the technician's hand to aid in feeling for imperfections. Note that the technician is using his left hand, his non-dominant hand, which helps to feel the imperfections more readily.
Following cleaning (both with compressed air to remove the dust and a final cleaning to make sure no contaminants are on the surface), the repaired area is ready for priming.
If the tools that you're using are cleaned, maintained and stored properly, they will last for years – perhaps your entire career. To keep your tools in good operating condition, air blow and wipe them clean, oil them and store them properly.
If the filler is mixed properly, to the right consistency, using the proper amount of catalyst, it can be applied quickly and smoothly with a single application. Pinholes can be kept to a minimum if the proper amount of catalyst is used, and by mixing or kneading correctly to remove the air.
Often new technicians do not realize when it is time to stop sanding and to apply another coat of plastic. By using a guide coat to help visualize the removal of scratches and low spots, and by using a trapped cloth between your non-dominant hand and the surface, it's easier to recognize when you should stop sanding. Learning how to apply plastic body filler properly, with practice, can keep the sanding to a minimum and help speed up the repairs.
