200 Section 2 Nonstructural Repairs Copyright Goodheart-Willcox Co., Inc. This procedure will abrade the surface, allowing the filler to bind to the rough surface. • Some manufacturers recommend that filler be applied over epoxy primer. The bare metal is sprayed with epoxy primer. Once the epoxy primer cures, it is scuffed and the filler is applied. Check the paint manufacturer’s recommended cure time. It may be as long as 24 hours. • The application site should be ground or sanded and then cleaned to remove oil, wax, or any other surface contamination. Figure 9-56 shows curing filler. Fully cured filler is sanded starting with 40-grit paper, followed by 80-grit paper and then 180-grit paper. On small areas that the sanding tool can span, start with 80-grit paper. Sanding before the filler is fully cured will clog the sandpaper with sticky filler residue. The filler can be tested for full cure by scratching it with a fingernail. If the filler feels soft, moist, or sticky, it is not ready for sanding. If the filler feels dry and hard, it is ready for sanding. The sanding process will level and featheredge the filler. Do not prime the filler for at least ninety minutes after it is applied. Solvents will evaporate from the filler even after it has cured. It takes ninety minutes for all the solvents to completely evapo- rate from the filler. Block sanding can be done with hand or power tools. In either case, the sandpaper surface should span the filled area whenever possible. This allows the undamaged edges of the panel to support the sand- paper, preventing the paper from undercutting the filler. If the filled area is too large for the sandpaper to span, you must sand the filler as evenly as possible in sepa- rate sections. The 40-grit or 80-grit sandpaper used to sand body filler can easily scratch the paint of adjacent panels. Protect the paint on undamaged areas with masking tape and cardboard. Block sanding levels filler. The goal is to apply one coat of filler and block sand it level. Too many techni- cians fill, block sand, find a high area, and tap it down. They then refill this area, block sand, find a low area, and refill again. Avoid wasteful refilling by identifying all high and low areas, applying an overfill of filler, and using proper block sanding techniques. Block sanding can be one of the most difficult auto body repair skills to learn. A common problem is oversanding the body filler, causing an undercut. An undercut occurs when the filler has been sanded so much that it is lower than the surrounding metal. The undercut must be refilled and block sanded again. Sanding the filler to match the surrounding contour, such as double crown or a body line, is another poten- tial problem. First, consider a 6” (15 mm) diameter section of body filler. A sanding stick can span this amount of filler. Hand placement on the sanding stick is impor- tant. Hands should be placed at either end of the stick and they should be supported by the undamaged metal around the filler. The undamaged metal will serve as a guide for block sanding and will prevent undercutting. See Figure 9-57A. Always follow the crown on the panel when sanding. Hold the sanding stick flat on low or medium crown. See Figure 9-57B. On high crown, hold the sanding stick on edge as shown in Figure 9-57C. If the sanding stick is held flat, the high crown will be sanded flat. When sanding double crown, the stick must be bent to follow one of the crowns, Figure 9-57D. On small areas, start sanding with 80-grit sandpaper. When a featheredge appears around the filler, switch to 180-grit sandpaper to remove the 80-grit scratches and to extend the featheredge. If there are no problems with the filler, such as pinholes, the filler is ready for the undercoat. Some body shops apply a thin coat of premium body filler or putty over the filler and into the surrounding featheredged paint, and then sand the cured putty with 180-grit sandpaper followed with 320-grit sandpaper. If pinholes or other low areas are present, they should be featheredged. Apply a layer of putty to the entire repair area, into the featheredged OEM paint. Figure 9-58A shows an edge that could not be featheredged because the area is still low. A low area remaining after block sanding is shown in Figure 9-58B. Once the putty has cured, sand it with 180-grit sandpaper. Some shops finish sand with 320-grit sandpaper. Areas larger than 6″ (15 mm) in diameter can be initially sanded with 40-grit sandpaper on an air file as shown in Figure 9-59. The air file will level the filler much faster than hand sanding. Try to sand the entire Goodheart-Willcox Publisher Figure 9-56. The first filler applied to this repair was short strand fiberglass filler. The possibility of small holes in the panel due to the extensive straightening efforts is why this type of filler was selected.