The method of constructing a hood is to use one sheet of metal and press it to shape on a structural web which is held to the outer surface of the sheet by a clinch fold lap joint. In some models of manufacture this overlap may appear to be filled with a mastic, but it is not. This filler material is applied after the pressing operation to make the clinch fold. The function of this mastic is to stop the paint on the raw edges of the metal overlap from breaking down and exhibiting early corrosion initiation.(reference)