Abstract: The present invention includes the use of a low density composite or foam core to support a high density rigid composite or foam layer or shell within a structural joint. The high density rigid layer or shell is located in areas immediately adjacent the joint welds while a substantial portion of the void in the joint is filled with a low density composite or foam core. A high density layer precursor may be molded onto the outside of a lightweight low density core. The reinforcement insert is dropped into the space between body panels before the panels are welded together to form a structural joint. The body panels with insert may thereafter be subjected to high temperature processes greater than 300.degree. F. causing the high density layer precursor to cure, further rigidify, and adhere the high density layer to the metal body panels to substantially stiffen the joint. The low density foam supports the high density layer precursor during the curing process.