When your plywood sheets aren't long enough, they must be joined end-to-end.
Butt joints aren't strong enough. Backing blocks can cause problems when you try to bend the panels. CNC-cut puzzle joints work great – if you have a CNC cutter. So the good old scarph (scarf) joint is the standard solution.
Here's how to make a plain scarph joint in plywood.
We start by marking off the width of the scarph, which should be 8 to 10 times the thickness of the sheet. For the 6 mm plywood used here, we use a 55 mm scarph. We'll flip one panel over, so that the two mating ends are facing the same way 55 mm apart, and screw or clamp both panels down temporarily. The resulting step forms the correct angle for our power plane.
We take a couple of passes with a sharp power plane. This is by far the easiest tool to cut these joints; a hand plane doesn't work very well and you can't hold most saws straight at such a shallow angle. (I've also seen some setups that use a large router bit and an angled router jig, if you feel like making some clever tooling.)
As we make more passes with the plane, the layered structure of the plywood provides a convenient visual cue. If the cut is flat, as it should be, the exposed layers will be straight and even. Waviness in the lines suggests places where you need to push a little harder, or back off the pressure a bit.
When we're satisfied that the joining surface is close enough to flat and straight, we can separate the two layers.
A flat substrate (in this case, a sheet of waferboard) is needed underneath the joint. We'll want a glue-release layer above that. (High-quality wax paper works well; other good choices are polyethylene sheeting or lightly buttered aluminum foil.)
The first panel is laid out flat on the substrate. The second panel, still upside-down for now, is in position beside it, waiting to be flipped over.
Our glue of choice is, naturally, epoxy. We start by spreading a thin film of neat epoxy on both faces to be joined. The end grain will quickly soak up this resin; we'll spread out the epoxy film again in a few minutes to ensure that the wood is fully saturated.
Now comes the epoxy that'll fill any gaps in the joint. This layer is thickened with colloidal silica to about the viscosity of ketchup.
We flip the second panel over, and line up the joint. Using scraps of plywood as pressure plates, we put closely spaced screws through the entire joint, right into the waferboard substrate. Starting at the centre, we work our way out to the edges with these screws, squeezing out excess epoxy as we tighten each one.
The next day, the screws can be removed and the release foil peeled off.
We've bent joints done in this fashion to a curvature of R=600mm (T x100) with no hints of failure.