198 Section 2 Nonstructural Repairs Copyright Goodheart-Willcox Co., Inc. pop out and crack the filler. Cracked filler will allow moisture to reach the metal, causing rust. A panel that has been previously damaged and repaired may be damaged again. If the existing body filler is thicker than 1/8″, it should be ground out and the low area should be raised. If the new fill area is adjacent to existing filler that is 1/8″ thick or less, the new filler can be blended into the existing filler once the paint is removed from the existing filler. See Figure 9-52. The first step in filling is to closely examine the repair area. The goal is to apply filler in a single appli- cation. Therefore, high spots must be eliminated before filling. Look for any high spots in the metal. Use a straightedge to locate highs. If highs are found, tap them down with a pick hammer. Next, remove the paint from the fill area. A grinder with a 24-grit disk may be used to remove the paint on high-crown steel panels. Grinding will leave deep scratches in the metal. The filler will anchor in the scratches. This is called mechanical adhesion. Because the heat produced by grinding can cause panel warping, use only 80-grit sandpaper on low- crown steel panels and all aluminum panels. The scratches created by sanding with 80-grit sandpaper will also anchor body filler by mechanical adhesion. Sand the OEM paint around the repair area to a taper. This taper is called a featheredge. The feather- edge is a gradual change in height from top of the paint to bare metal. This slope will hide the change in height, creating an invisible repair. NOTE Body filler can be applied over bare metal, cured epoxy primer, and sanded OEM paint. For corrosion protection, some vehicle manufacturers recommend applying filler over cured epoxy primer. Check the manu- facturer’s requirements. Follow the paint manufacturer’s requirements for cure time. The cured epoxy primer will need to be sanded with 320-grit sandpaper before filler application. Epoxy primer is required over silicon bronze welds and bare aluminum panels before filler applica- tion. If epoxy primer is required, spray two coats over the bare metal and into the featheredge. See Figure 9-53. Once the panel surface is prepared, mix the filler according to the manufacturer’s recommendations. Read the instructions on the can carefully. Mixing filler and hardener is a critical step in panel repair. Using the wrong amount of hardener will cause problems. The can of filler will list the proper mixing ratio. Always use clean mixing sticks, paddles, and mixing boards. Dirty equipment can contaminate the newly mixed filler. The mixing board must have a nonabsorbent surface, such as metal or plastic. Never use cardboard as a mixing board. One type of mixing board is a tablet of nonabsor- bent paper sheets. After a sheet is used for mixing, it is torn off the tablet, exposing the next clean sheet. Before mixing filler, make sure it is uniform in consistency. If a bluish liquid, which is the resin in the filler, is visible inside the filler can, use a clean paint stick to mix the resin back into the filler. Use a clean paint stick or paddle to remove the desired amount of filler from the can and place it on the mixing board. Put the lid back on the filler can as soon as possible. An open can of filler can be easily contam- inated with the sanding dust present in the shop. Paste-type hardener will separate inside the tube. Knead the tube of hardener to thoroughly mix the Goodheart-Willcox Publisher Figure 9-52. The panel contains body filler from a previous repair. Notice the rust that has formed under the filler. Filler Primer Goodheart-Willcox Publisher Figure 9-53. This damaged quarter panel required raising, leveling, and shrinking. A shrink fence is visible.