Repairing Grinder Marks in Sheet Metal

Sanding

• Sanding scratches the surface of sheet metal to remove the grinder marks. The grinder marks are essentially scratch marks made from a coarse abrasive grinding disk. Sanding requires several steps, beginning with a coarse-grit sandpaper, such as 80-grit, that leaves finer scratch marks than the grinder marks. A medium-grit sandpaper, such as 120-grit, removes the coarse-grit scratch marks, replacing them with even finer scratch marks. Sanding with progressively finer-grit sandpaper continues until the desired smoothness is achieved. Hand sanding is practical for small areas, but a dual-action electric sanding tool produces more efficient results on large grinder marks. Always sand with the grain of the metal and keep power sanders moving constantly so the metal isn't burned.

Body Fillers

• Body filler may be used to repair rust holes or along weld beads if a furrow is left along the bead after grinding and sanding. Body fillers cover the metal, which means they also cover the grinder marks and fill in small pits in the surface of the sheet metal. The two main types of fillers used for sheet metal are lead and plastic body fillers. Plastic body fillers are smoothed onto the metal in paste form, using a plastic applicator. Lead fillers are applied in hot liquid form and guided into the crevices with wooden applicators. After the chosen body filler has set according to package instructions, sanding is required to smooth the surface.

Primer and Paint

• Proper application of primer and paint to the metal surface conceals the repaired area, helping it blend with the rest of the surface. Use a self-etching, metal primer to cover the sanded area and any body filler. Self-etching primers fill in any fine scratches that remain on the surface, helping the paint to bond and concealing the scratch marks. They also act as a moisture barrier. After the primer has dried, paint the surface the desired color with a paint designed for metal surfaces and seal it with a clear coat, if desired. Priming and painting are essential to the long-term effectiveness of the repair because they prevent moisture from rusting the sheet metal.

Polishing/ Buffing

• Polishing is a method used to restore luster to bare metal surfaces, such as chrome, nickel, copper and brass. Use this procedure only if the grinder marks have been sanded and no body filler or paint has been applied. Polishing is done either with a clay bar or a dual-action sander with a soft cloth pad attachment. If using a clay bar, the bar itself acts as the polishing compound. With a dual-action sander, a liquid compound is applied to the metal, and the soft attachment works the compound into the surface.

  If body filler and paint are applied to the metal, buffing is used to give luster to the paint. Use a buffing pad on a dual-action sander to buff the paint. Apply wax to the pad, and work the wax into the repair area with the dual-action sander. For best results, buff the entire metal surface so the repair area blends with the rest of the metal.