Which method is appropriate for repairing shallow scratches in sheet metal?

Burnishing is the appropriate method for repairing shallow scratches in sheet metal because it involves using a tool to smooth and polish the surface of the metal. The process compresses the metal around the scratch, effectively reducing its visibility and improving the overall surface finish. This technique is particularly suitable for shallow scratches, as it does not require the additional material or structural changes that methods like filling might entail.

In contrast, filling is typically used for deeper imperfections where material needs to be added to restore the surface profile. Welding involves melting and fusing metal, which is more suited for joining pieces or repairing significant damage rather than addressing superficial scratches. Riveting involves fastening two or more pieces of material together, which is not relevant for a simple surface flaw. Thus, burnishing stands out as the most effective and appropriate technique for this type of repair, specifically targeting shallow scratches without compromising the integrity of the metal.