If you have any unevenness on the scarf joint be sure to sand the surface with a hard and flat sanding block. For best results keep the plane blades sharp at all times. Then use a more finely set block plane to finish off the scarf bevel, being careful to keep the bevel straight and flat as you proceed. Using a power plane or belt sander can save a lot of time in roughing the scarf bevel down close to its intended dimension, but without this luxury use a smoothing plane with a sharp, deep-set blade to remove a majority of the stock as quickly as possible. Making a scarf joint by hand, the basic tools require a block plane and a smoothing plane. The disadvantage is that fabricating a scarf joint will take more time and skill compared to the butt block and also reduce the length of the plywood. In demanding situations, extra strength can be gained by increasing the length of the scarf and they are even less likely to produce a hard spot that could result in an uneven surface when bent. The advantage of scarf joints is that they are strong, lightweight, and reduce stress concentrations. For example, when using 1/2″ (12.5 mm) plywood you need a 4″ long bevel. In many boatbuilding situations, we recommend a ratio of 8-to-1 to determine the size of the bevel. Dry fit these matching bevels, and then permanently bond them with WEST SYSTEM® Epoxy. Avoid overlapping the pieces of plywood.Īlthough there are several different kinds of scarf joints, we advocate using the simplest and most reliable approach, fabricating identical bevels at the ends of the pieces of plywood to be joined. Scarf joints are tapered to reduce concentrated stresses. Increasing the wood thickness on the glue joint adds more stiffness which creates a hard spot when the panel is bent around a curved form.ĭepending on the application, a butt block joint may be adequate. The width of the butt block should be a minimum of 16 times the thickness of the plywood you are using.Īlthough a butt block joint is a fast way to joining plywood, it does have many disadvantages:īacker blocks create stress concentrations because “the block” is only reinforcing one side of the joint creating an imbalance.Įxposed end-grain on the butt block is susceptible to moisture absorption unless it is coated with epoxy. A butt block joint is made by edge gluing two pieces of plywood together and adding a backer block to one side of the seam. Butt Block JointĪ wooden butt block joint provides a simple way to attach two adjoining pieces of plywood together.
Each method offers its own distinct advantages and disadvantages in certain applications so it’s important to be aware of each joining method. The most common methods of joining plywood are the butt joint with backer block, the scarf joint and a hybrid between the two. Many boat parts require plywood lengths greater than the standard 8 feet, so joining together two panels of plywood is an important step.
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