Before finishing a fiberglass box, it needs to be sanded. The best way to sand a fiberglass box would be using a sanding wheel as opposed to manually sanding it by hand. As you are most likely aware, sandpaper comes in a variety of grits making some sandpaper rougher than others. The ideal sandpaper for this job is one with an 80-grit rating. When sanding, particular attention should be paid to the resin area. Sanding will not be complete until the resin buildup is sanded down to the same thickness as the rest of the box. Final tip: When sanding, make sure you are wearing a mask to cover your nose and mouth to prevent inhalation of fiberglass particles.