Face grain of patch parallel to face grain of skin Pressure plate  1 ∕ 8 " or ¼" plywood Waxed paper or plastic wrap Weights or clamp Trim to circular shape (15T maximum diameter)  Minimum distance to frame = 15T T =  1 ∕ 10 " or less Plywood skin 5T 5T Figure 6-29.  Splayed patch. Plug Patch Two types of plug patch, oval and round, may be used on  plywood skins. Because the plug patch is only a skin repair,  use it only for damage that does not involve the supporting  structure under the skin. Cut the edges of a plug patch at right angles to the surface  of the skin. Cut the skin also to a clean round or oval hole  with edges at right angles to the surface. Cut the patch to the  exact size of the hole; when installed, the edge of the patch  forms a butt joint with the edge of the hole. You can use a round plug patch where the cutout repair is no  larger than 6-inches in diameter. Sample dimensions for holes  of 4-inches and 6-inches in diameter appear in  Figure 6-31. The following steps provide a method for making a round  plug patch: 6-21 4. Cut a doubler of soft quarter-inch plywood, such as poplar. A small patch is cut so that its outside radius is 5⁄8-inch greater than the hole to be patched and the inside radius is 5⁄8-inch less. For a large patch the dimensions would be increased to 7⁄8-inch each. If the curvature of the skin surface is greater than a rise of 1⁄8-inch in 6-inches, the doubler should be preformed to the curvature using hot water or steam. As an alternative, the doubler may be laminated from two pieces of 1⁄8-inch plywood. 5. Cut the doubler through one side so that it can be inserted through the hole to the back of the skin. Place the patch plug centered on the doubler and mark  around its perimeter. Apply a coat of glue outside the line to the outer half of the doubler surface that will bear against the inner surface of the skin. 6. Install the doubler by slipping it through the cutout hole and place it so that the mark is concentric with the  hole. Nail it in place with nailing strips, while holding  a bucking bar or similar object under the doubler for backup. Place waxed paper between the nailing strips  and the skin. Cloth webbing under the nailing strips facilitates removal of the strips and nails after the glue dries. 7. After the glue has set for the installed doubler, and you have removed the nail strips, apply glue to the inner half of the doubler and to the patch plug. Drill 1. Cut a round patch large enough to cover the intended repair. If applicable for size, use the sample dimensions in  Figure 6-31 . The patch must be of the same material and thickness as the original skin. 2. Place the patch over the damaged spot and mark a circle of the same size as the patch. 3. Cut the skin inside the marked circle so that the plug patch fits snugly into the hole around the entire perimeter.