I finally finished the fin. After final sanding and epoxy coating, it's weight increased to just 860 grams (11 grams of epoxy coating, about 1/3 ounces). The photo shows the weight with full metal hardware on, including hinge halves, mounting brackets, bolts, nuts washers. It weighs 1085 grams (38.3 ounces). So far, I'm happy with it.
