When thin sheet metal is constrained at its edges or subjected to uneven forming stresses, the center bulges out slightly. Pressing on the bulge causes it to pop inward with a distinct metallic sound, much like the bottom of an old-fashioned oil can. It is a common aesthetic defect in metal roofing, ductwork, and automotive panels.
Heard on the job
We need to add a stiffening rib down the center of this panel to stop the oil-canning.
Insider tip
Thicker gauge metal, matte finishes, and breaking up flat spans with ribs or slight curves are the standard ways to hide or prevent it.