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Archive for the ‘Joinery’ Category

1. The Intro: Why Scarf Plywood?

Scarfing plywood is an essential skill for any boat builder. Plywood comes in larger sizes than lumber simply sawn from trees, and is more dimensionally stable. Plywood can also be sealed. On the other hand, wood planks shrink and swell and are much more difficult to seal. Because plywood comes in standard 4ft by 8ft sheets in various thicknesses, we must glue them together to get the length required for most boat building applications. There are many reasons why the scarf is ideal for joining sheets of plywood. Four main reasons are:

  • the scarf’s strength while bending and flexing the wood
  • long glueing surface.
  • clean look in the finished product.
  • and low difficulty with common boat shop tools.

2. Types of Scarfs

12to1scarf

A 12 to 1 scarf ratio

Scarfs are mainly differentiated by the ratio of the length of the scarf to the thickness of the plywood. For example, the above scarf is cut into a 1 inch thick sheet of plywood and has a 12 inch long scarf. Most scarfs in boat building are 8 to 1 scarf ratios. However for structural applications that will encounter elevated stresses, a 12 to 1 scarf is in order. Gougeon Brothers increasingly recommend the 12 to 1 scarf ratio instead of 8 to 1 to obtain the strongest joint.

2. Cutting the Scarf

Notice the straight lines created by the separate plies.

An excellent article on cutting the plywood for the scarf can be found here.

It is important to cut your plywood scarf down to a feather edge on the end. If not, you will create a bump at the joint. To fine tune the scarf, first layout your plywood on a flat workbench, so the end of the plywood with the cut scarf edge is flush with the end of the table. Moving the scarf to be tuned all the way to the edge of the table but no further allows you to plane to a tiny feather edge while not damaging the edge.

Next, place a line on the face of the plywood marking the end of the scarf. You can now use a sharp block plane to remove the wood. It is easy to plane the scarf accurately by keeping the lines from the different layers of plywood straight.

Hand Planing Plywood Scarfs

Hand Planing Plywood Scarfs

You may need to sharpen your plane often, as most marine plywood will dull the edge of your plane very quickly.

3. Glue Basics

If you are experienced in working with epoxy, you can skip this section.

In this article I am using West System epoxy. In order to produce a good glue joint, the following principles should be followed.

  • Always mix the 105 epoxy resin and the 206 hardener thoroughly before adding fillers.
  • Always wet out end grain, especially on plywood, as it soaks up much more glue than solid sawn lumber.
  • When you clamp the two surfaces to be glued together, verify that glue squeezes out all along the joint to make sure you have enough glue for a strong, solid joint. While this can create a mess and waste some epoxy, insufficient epoxy can be much more costly if the joint breaks later.
  • It is a good idea to wear a respirator with a carbon filter and eye protection while working with epoxy. Resin in the eye is extremely uncomfortable.

4. Gluing up the scarfs

Now that the scarfs have been cut, we can glue the two sheets together. First, fasten wood cleats to the floor. Next fasten the two sheets of plywood to these cleats so the sheets fit together the way they will be glued. Most methods do not involve the extra step of elevating the scarfs on cleats. However, this ensures that when the thickened glue squeezes out of the joint on the bottom, it will not get trapped between the sheet and the floor, causing a hump at the scarf joint.

One plywood scarf is elevated.

One sheet's scarf is elevated, and the end grain is being wet out.

Now you are ready to begin wetting out the plywood. Mix up some clear epoxy (no filler), and brush it on the scarf. After the endgrain on the scarf  quickly soaks it up, apply more. This may take a half hour or more, coming back every 10 or 15 minutes between applications.

Once you have saturated the plywood with epoxy, mix a batch of epoxy with 406 and 403 fillers to the consistency shown in the picture and brush it on both scarf edges. Too thick a mixture could result in insufficient squeeze-out from the joint creating a hump at the joint.

Epoxy consistency for scarfing plywood

Notice the consistency of the epoxy.

After applying the glue, simply remove the wood that was elevating the top scarf and carefully lower the top scarf on the bottom scarf. To ensure good clamping pressure, fasten a board on the top of the joint, but not on top of where the squeeze-out comes out.

Soon, your scarf will dry and you’ll be ready to move on to the next stage in your project!

5. References

Applying Thickened Epoxy to Plywood Scarf

Applying Thickened Epoxy to Plywood Scarf

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