The edge line of sheet metal is not obvious.

I want to ask: what is the reason why the edge line of sheet metal is not obvious? The reason for this is the following：

1. Corrosion or damage: The rib line of sheet metal is corroded or damaged, which will make it inconspicuous, which is caused by too long use time, the influence of external environment or improper maintenance.

2. Surface coating or renovation: The surface coating or renovation of sheet metal, such as painting and electroplating, will cause the ribs to be no longer obvious.

3, quality problems: defects or imperfections in the manufacturing process, ribs will not be obvious or clear.