gussets. When the damage follows or extends to a framing  member, support the scarf as shown in section B-B. When  the scarf does not quite extend to a frame member, support  the patch as shown in section C-C.  Damage that does not exceed 25 times the skin thickness  (3 1 ⁄ 8 -inches for  1 ⁄ 8 -inch thick skin) after being trimmed to  a circular shape can be repaired as shown in section D-D,  provided the trimmed opening is not nearer than 15 times  the skin thickness to a frame member (1 7 ⁄ 8 -inches for  1 ⁄ 8 -inch  thick skin).  A temporary backing block is carefully shaped from solid  wood and fitted to the inside surface of the skin. A piece of  waxed paper or plastic wrap is placed between the block  and the underside of the skin. The scarf patch is installed  and temporarily attached to the backing block, being held  together in place with nailing strips. When the glue sets,  remove the nails and block, leaving a flush surface on both  sides of the repaired skin. To repair a section of the skin with a scarf patch when access  to the back side is not possible, use the following steps to  facilitate a repair, as shown in  Figure 6-34. Cut out and remove the damaged section. Carefully mark  and cut the scarf around the perimeter of the hole. Working  through the cutout, install backing strips along all edges that  are not fully backed by a rib or spar. To prevent warping of  the skin, fabricate backing strips from soft-textured plywood,  such as yellow poplar or spruce, rather than a piece of  solid wood.  Use nailing strips to hold backing strips in place while the  glue sets. Use a bucking bar, where necessary, to provide  support for nailing. A saddle gusset of plywood should  support the end of the backing strip at all junctions between  the backing strips and ribs or spars. If needed, nail and bond  the new gusset plate to the rib or spar. It may be necessary  to remove and replace an old gusset plate with a new saddle  gusset, or nail a new gusset over the original. The Back of the Skin Is Not Accessible for Repair Unlike some of the other type patches that are glued and  installed as one process, this repair must wait for the glue to  set on the backing strips and gussets. At that point, the scarf  patch can be cut and fit to match the grain, and glued, using  weight for pressure on the patch as appropriate. When dry,  fill and finish the repair to match the original surface. 6-26