The next morning, we roll the car back to the dirty side of the shop and are ready to lay on filler. Auto body filler, commonly called by its original trade name of Bondo, has come a long way from the original Bondo product. Modern filler is strong, lightweight and easy to sand, with minimal shrinkage, which was the serious problem with the earlier stuff.
With a coat of primer on the car, areas needing filler will be evident. This will mostly be in areas where damage has been repaired, patches have been welded in or possibly where the factory did not do a very good job (gasp!). Filler is comprised of polyester resin, and lightweight thickening agents, such as glass micro-spheres. The higher quality fillers use better and more stable ingredients, leading to easier working, smoother application and better long-term dimensional stability. They also cost more.
The desired amount of filler is scooped out of the can it comes in onto a mixing board rather like an artist’s palette. A measured amount of catalyst is added, usually a colored cream containing benzoyle peroxide. The color is there to let you know when the filler and catalyst are properly mixed. Then the filler is spread on the area to be built up, using a variety of flexible or stiff spreaders, as the case requires. One must work quickly, since the filler has a limited working time. Once it begins to “gel”, no further spreading is possible, and the filler is left to set.
There is a narrow window, when the filler has set, or gelled, but has not completely hardened, when a quick bit of shaping can be done. A tool similar to a Stanely Sureform file, commonly referred to as a cheese grater, is used to quickly pare down the filler close to the desired shape. Once hard, the filler is shaped further with coarse sandpaper. If the low spot is not completely gone, additional filler can be layed on. This process continues all around he car until the basic bodywork is finished.
At this point, a smoother, lighter filler, called glazing putty is spread over the areas where filler was employed. Glazing putty is fine-textured and goes on like icing. In fact, one of the brands of glazing putty is called “Icing”. Glazing putty will fill any pin-holes in the filler or any rough sanding scratches that may be left. The putty is catalyzed, just like the filler, smoothed on and sanded down, generally to 180-grit at this point.
By now, the car is covered with filler and putty and may even have primer sanded off on a few high spots. It is also very difficult to judge if any flaws remain, due to the various textures and colors. To move beyond this point, we need to re-establish a reference point.