Copied from
Old School Model Aviation

"The dihedral angle is set by propping up each panel 1-1/4 inches at the tip, holding a sanding block vertical to the work surface, and gently sanding each center rib. Trial-fit the wing panels, sanding again as needed, and then pin down one panel while raising and supporting the other panel 21/2 inches at the tip.

Thick CA or epoxy can be used to glue the two panels together. Sand the wing, and then using thin CA adhesive, glue a 5/8-inch strip of lightweight fiberglass over the seam to strengthen the dihedral joint."