I have no excuse to offer, but I fail at plywood scarf joints. Other builders seem to have no problem with using a block plane (I’m looking at you, Jon!), but my skills are not sharp enough for that (neither are my tools, apparently). Thankfully, you can do this electrically with a router. (I've seen homemade circular saw jigs, but never tried them).
Warning: this is extremely loud and unpleasant. You will also generate obscene amounts of sawdust.
The idea is to build an angled track that the router will rest on. As you move the router, the bit will gradually cut through the wood at an angle, forming that perfect triangular taper.
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The jig consists of two triangular pieces of wood cut to the appropriate angle depending on your scarf joint. For joining plywood, this is probably 8:1. So, measure some scrap plywood 16” (or more) wide, and 2” (or more) high at a right angle. Connect the lines and cut out your triangle. Now make a duplicate. Use blocks of wood to connect these triangles at the high end so that they are held maybe 5” apart or so. Base this distance on your router size: you want the router to be able to move a bit left-or-right in the channel, but not ever fall off either support. Narrower is better than wider. Make sure to not use any supports near the sharp pointy end of the triangle, since that is where the router will be cutting.
Measure 2” back from the edge of your plywood and draw a straight line across. This will be the start of your scarf joint. Place the triangle jig on the wood so the pointy ends are hanging off the edge maybe an inch or two. Put the router on the incline and make sure that the router bit can lower enough to extend so that it will cut through all the plywood. Then, securely clamp a straight piece of wood to the plywood to act as a rear fence for your triangle jig. This will keep the jig in the correct orientation and prevent it from sliding backwards, which would ruin the calibration with your router bit depth. Make sure this fence is parallel to the edge of the plywood.
Press the triangle jig firmly against this fence and place a heavy weight on the rear part to hold it down. I used lead blocks. Then, place the router on the jig and lower the bit so that the tip of the bit just touches the plywood where the pencil line is marking the start of your joint. Since the incline of the jig is 8:1, the router bit will then exit the wood at the very end of the piece. Setting the bit deep enough is very important, especially if you are stacking multiple pieces of ply. Your bit needs to cut completely through one piece of ply. If you are not deep enough, your scarf length will not be 8:1, thus weakening your structure. OPTIONAL: If you want, you could raise your bit slightly so that you leave one ply or a half ply of wood at the end of the plywood. This will prevent tear-out of the plywood and give you a nicer edge in case you are varnishing the boat. I was painting, so just filled the tear-out with thickened epoxy, but the joint IS ugly. Keep that in mind. If you leave a thin edge, you must go back over the joint with a block plane or sander to remove the extra material - make sure to remove the same amount of material from across the entire joint so that the incline is consistent and not curved. Curved would be bad.
Now that your router bit is set to the correct depth and the jig is held securely in place, turn on the router and carefully pull it down the incline. Move the router back to the top and repeat. Do this until you get close to the edge of your jig, then move the jig over and continue. This will take a while. The edges are especially tricky since one of the jig sides won’t have anything to rest on. In that case, just do the best you can and gently lift the router up so it isn’t pressing down hard on the jig.
Of course when you are done, you’ll still need to run over everything with sandpaper.
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