What is the main purpose of using a straight file on sheet metal?

Using a straight file on sheet metal serves primarily to identify high and low spots on the surface. This process is important in achieving a smooth and even exterior finish, which is crucial in non-structural repairs. By filing the surface, technicians can effectively feel for unevenness, as the file will remove material from the high spots. This tactile feedback allows for a more accurate assessment of the metal's surface, ensuring that any necessary corrections can be made to achieve the desired contour and smoothness before moving on to tasks like painting or applying filler.

While other options may relate to functions performed on metal, they do not specifically address the primary purpose of filing. Flattening the metal is usually achieved through other methods, such as hammering or using a dolly for shaping. Preparing the area for filler and sanding are also tasks that may involve different tools or techniques better suited for those operations, separate from the direct use of a file for surface assessment.