Background
Truck wall and body shell join angle isn’t 90°
The top of the truck bed is surrounded by aluminum L-shaped caps that hide the join between the bed wall (and tailgate inner wall) and the fiberglass shell (and tailgate skin).
To complicate matters, in most cases the wall and shell join is not at exactly 90 degrees; in fact, the fiberglass tends to bend down to meet the top of the aluminum wall causing an inevitable air gap between the cap and the shell/wall surface. This gap could be filled with bondo, but this would be a laborious task.
If the caps are glued (3M panel bond or window weld as suggested by Smyth Performance), there isn’t a non-invasive method to force the caps completely down against the fiberglass and wall surfaces. Thus there is a risk that gaps will appear along the rail edge.
An alternative fastening method, illustrated in the second drawing, uses 10-24 machine screws; an added advantage is that the top caps become removable. Machine screws mitigate the gap problem for a more consistent installation.
A further refinement would be placing rubber edge trim along the 1/8″ aluminum cap bottom edge to cushion it against the truck bed wall.
A rubber trim also acts as a buffer to eliminate rattles caused by the unsecured bottom of the cap hitting the side of the truck bed; trim also effectively “closes” the air gap between the wall/shell/cap join.
